I don't mean "sitting down to write", I mean the literal beginning. The story itself starts with my protagonist running. I need to get him to run, then I'll have the rest of it sorted. But won't starting with "He ran" sound cheap? Should I add something before? Or should I just go straight with "He ran. This and that happened, so now he's being chased"? Or maybe it would be better to start with describing "this and that" and then progress to the running sequence? The "this and that" wouldn't have much to do with the story, so I worry whether it won't be just a filler and won't bore the reader. That's what I'm worried of the most, that I would bore them before anything would start.
The opening lines generally set the tone of the book. Why are you starting with your protagonist running? Is that the theme of the story, are they running away from something physical or emotional?
Take the first line from Pride and Prejudice:
"It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife"
That is indeed the main motivation of Bingley, Mr Colins and even Darcy and Wickham. It's also the preoccupation of (most) of the female characters. The opening line sets the expectations of the reader for what is going to follow.
So my answer is to think about the overall theme of your story and write a beginning that informs the reader of the journey they are about to embark on. The art is doing that without being blatantly obvious about it.
The first part of a book / story should almost always be the normal life before the event that really pushes the story forward.
If you don't want to do that, you need to ask yourself why he's running. If he's running for a reason (emergency, he's in danger etc.) then "he ran" is absolutely fine.
You can also decide how much information you want to give at this point. Saying something like "Looking over his shoulder again, he ran. His breath, laboured as he struggled to fill his lungs while his suit jacket flapped around him." You reader knows he's running and he's scared but doesn't know what he's running from. Or towards.
Equally, you could start the book where he doesn't want to run (over did it at the gym last night and his legs are really hurting today?) but by the end of the chapter, something else has happened that means you can end that chapter with "he ran."
Disclaimer: this answer relies on my own experience and may not fit your needs.
If you story starts with someone running, then there must be a lot of action later.
Even if it's wrong, this will be the first assumption of the reader while reading it. I will not go in detail about this because Matt Hollands' answer is already covering it pretty well.
The same way, if you begin a book setting a date, let's say "2014", you unconsciously know the story will stretch on months or even years. But if you start with "monday", your whole story won't probably last more than a few days, which set different expectations to the reader.
If you begin In Medias Res, start answering question early instead of piling them.
Years ago I wrote a series of short stories that somehow get published in a small magazine, and the first one literally started with "He ran" ("He" being the name of the protagonist).
At the time, my litterature teacher told me it worked because within the next sentences, I was explaining the threat from who he was running from (who was after him and why), and the setting which was important for the rest of the story (empty streets after a curfew, troops of Guards looking for offenders, wanted posters with his face on it, etc.).
The important thing here is: your "this and that" must not be a filler. If your character is running, there is a reason for it and your firsts paragraphs must expand on that reason. If it is unrelated to what follows in the story, then your MC running may not be the good approach.
Then, when the threat is gone temporarily, the protagonist may rest and have a flashback about how he got there (probably not the best follow-up, but that's what I did at the time).
Now the whole "You probably wonder how I got there"-flashback thing may sound silly, but it is exactly what you see or read in most of action movies nowadays. It is the stinger, or the hook of the story. A little digest of what could happen later: action and mystery. Your "hook" reflects what the reader will find later in the book; if the reader adhere with your first chapter, you already know he will read the rest.
Lots of fine answers here, and as many good choices for you. Starting with the running is great stuff if that's what your book is about. 'In medias res' is the formal phrase for it, and that tells you how often it happens.
But don't underestimate the moments before the running starts. It doesn't have to be boring, it can definitely be used to ratchet up the tension. Maybe he spots one of his pursuers, but he doesn't know what the others look like. He tries to blend in with a group and casually exit, but then runs into the partner.
Or maybe he's part of a group watching a dead-drop to see who collects the bundle of cash, and he's on the coms with his own partner. Then a maintenance truck blocks his view of the park bench, and by the time he gets through a crowd of people, the backpack is gone. He spots two people running in different directions!
The point is, the reader knows weapons are about to be drawn, and the running will start any moment. You have a chance to define good guys and bad guys, and display a bit of your protagonist's personality while building tension. You're just tightening up the spring until it snaps and the running starts.
You don't have to just say "He ran", your opening sets the stage for where the action takes place and gives us insight into the character. You can state things about the environment, what the character is thinking as he is running, what the character looks like or what their background may have been like before they had to run.
Shadows danced over the graffitied walls and dumpsters of the alleyway as he ran past streetlights and illuminated windows.
It seemed as though the shadows themselves were chasing him as he ran through the graffitied walls and dumpsters of the alleyway, no matter how fast he ran the shadows kept up.
His tailored suit flapped wildly as he ran along the graffitied alleyway, his slicked back hair disheveled from his frantic running.
My suggestion is to view your story like it's a movie. Imagine they're not reading, but WATCHING. How do you want your readers to visualize the scene? If you start the story in Media Res, put MC right in the middle of the chase scene, and really zoom into the action- he's jumping over an apple cart, apples are flying everywhere, he landed in a crate of pears, the vendors are screaming- but he can't stop! Then zoom out later when the action dies down and explain what happens.
You could also do a POV shift: tell your readers about what's going on by placing them next to someone telling the story- the bad guys, for example, explaining what they're gonna do to MC when they catch him, finally getting their revenge for that time when he did XYZ- then pan back to MC.
Or, you could do an alternating sequence, either with MC before the chase in a state of calm alternating back and forth between this and the chase, or with MC after the chase in whatever state he's in, alternating to the chase.
In essence, just play with structure! There's no reason for things to be in order if the reader will figure them out eventually!
It sounds like you are grappling with the finer points of opening In Medias Res - right slap bang in the action. This is a tried and tested opening move.
What you will be looking to establish right away is the source of the menace (threat). Every scene you write must answer the question, "why should I care?" To do that you must establish what the character's objective is and the problem stopping them from getting it.
Aristotle's theory of drama (very roughly) starts with pity, then struggle, and final catharsis. Your opening is all about us, as readers, having an empathetic reaction to the misfortunate's plight. Show us that.
In an action scene, the objective is either get a thing or get away from something. It sounds like the problem for your character is one or more persons are trying to catch, kill, or hurt the character. Once we know the danger itself, we will care about how they solve they get out of that situation (at least for now). Only then will we start to think about why they were in danger and what they will do about that.
Sure, open with "He ran." It packs a lot of information into a very small package. Just so long as the very next thing you tell us is what he ran from or why he ran. Give us reasons to root for the runner. Centre us in the runner's immediate problem.
He ran. He loved running. Every morning, those two hours were his only escape from family life.
Is a very different story to:
He ran. The wolves were right behind him. Unless he could find somewhere to hide, he would be lunch.
In the first example, we want to know what is so bad about his family that the guy needs to escape every day. Once we know that, we will be willing to learn how that particular run started and what made it different - why this one matters. These two facts feed into what the character will do next.
In the second example, we want to know how he gets away from a pack of hungry animals. Unless why they are chasing him forms part of how he gets away, we will not care why until later. Once he escapes, we may care about him enough to learn how he ended up getting chased. Those facts feed into what he will do next and take us with them.
Generally, do not start with a pronoun. Start with a MC name. Other than that, sure, open with "Jack ran, [Why Jack is running, something about the scene]."
On the first page readers are forgiving, they are aware they know how stories work and that not all information can be dumped at once. They expect to understand within 5% of the story, but in the beginning you don't have to explicate everything at once, and if you do it is boring.
The opening is where you deliver the most important things about your MC, so talk about him, his thoughts, his feelings, and indirectly teach the reader about the world through his eyes and experience, but slowly, avoid information dumps. Devise your opening so the MC is active and doing things, and preferably not in a way that will require a flashback later (although references to the past in dialogue are fine, the MC explaining or reporting something to somebody else).
Let us follow the MC doing whatever he is doing, and you make sure that what he is doing reveals (by showing, not telling) elements of his character for us to latch on to, sympathize with him or root for him.
Jack ran through the woods, looking for a tree tall enough to use as an ambush station, ...
Jack ran, sprinting down the street at dawn. He was ten minutes late, and if he didn't catch the school bus on Kennedy street he couldn't save a seat for Karen, and then he might as well not have lived this day. Jack ran.
Add character-specific tension in the first sense, it isn't enough to be running or fighting, show us something the character wants or needs from the beginning, tied to the main plot or not, but tell us something about why the MC is running and something about his character. The first Jack above is a likely a soldier planning to kill an enemy (or enemies).
The second Jack is a schoolboy in love with a plan and a minor problem to solve, running late. That is unlikely to have a huge impact on the overall plot, but it tells us something about Jack and his setting, so we (readers) are semi-anchored. It doesn't have to be much. As long as we have a pretty good idea of the time-period and setting and who we are following (the MC, or first MC introduced), we'll keep reading. Both sentences above are enough to buy you (the writer) a few pages because they raise questions for the reader; "what happens next?" Does Jack catch the bus? While they wonder that, Jack can think so we know him better, or pass things on his way so you provide more of the setting, etc. But we keep reading because Jack has a goal and we immediately want to see Karen and see Jack interact with her, on the bus.
Do NOT get bogged down in backstory or the setting or Jack's "situation" or big problem, that can all dribble out later. The inciting incident is 10%-15% into the story, this first 10% is showing us what the MC's normal world is like, then we encounter the inciting incident, and by 25% the MC is leaving the normal world (metaphorically or literally) to solve the problem presented by the inciting incident, which has escalated by 25% to something the MC can no longer ignore or solve with a simple fix.
This is how stories start, not by any rules set by any authority, but when we analyze successful stories the vast majority follow this pattern: Normal World (10%), Something Happens (5%), It Gets Worse (5%), We must leave our normal world to deal with it (5%).
More details in my answer to Where Do I Start?