Before solving a problem, see if you can remove the problem -probably somebody at some point
I don't think you need to exercise this writing muscle, IF you can change the writing approach you take. You seem to be trying to plan and then write, something a lot of people do because they think it is necessary. It's not. Most of the time my only guide is 'have this guy, do something cool, happy ending', and sometimes I don't even have that.
I'd recommend moving from a plotter approach to a more pantser style. You probably don't know what either of those mean, so quick explanation before I continue; Plotters write out the plot points, then write based off of that to get what they eventually end up with. Pantsers on the other hand, don't plan at all and just go. Personally, I'm a pantser, and am planning to publish this year.
Doing this will let you write without having to worry about what comes next, because you'll decide that as it comes. Outlines become optional, and things just appear as you decide what works well.
Anyway, if you remove the need for an outline, you also remove the need to get better at doing it. Don't think that being a pantser is going to make the writing less good, I'm one and my writing is fine, and JK Rowling and Stephen King are also pantsers.
Good luck writing.