I am not a lawyer. But ...
I don't know how police and courts treat an autobiography. I've never heard of someone being convicted of a crime because he confessed to it in an autobiography. But a very common method for police to catch a criminal is that he tells someone else about the crime and that person reports it to the police. Many criminals seem to get caught because they foolishly brag about how they got away with it to their buddies at the bar. And I've seen several stories in the news lately about people being arrested after making posts on Facebook and the like describing a crime they just committed. So I wouldn't just assume that something you put in a book "doesn't count".
As others have noted, a lot depends on the seriousness of the crime. If you write that 20 years ago you exceeded the speed limit by 2 miles per hour, I can't imagine that the police or courts would care enough to track you down on this. On the other hand, if you write that you were the person who committed an unsolved murder, then I think the authorities will be interested.
You mention committing various (minor) illegal acts while travelling. That means we are talking about the laws of not just one country, but many. Even if a lawyer assured you 100% that mentioning in your book that you smoked marijuana in the United States won't get you into trouble, maybe it will get you in trouble if you say you did it in Saudi Arabia or Singapore and then return to that country.
I wouldn't be too encouraged by the fact that some celebrity confessed to drug use in a book and nothing came of it. I don't want to get off into politics, but I think reality is that the rich and famous get away with things that ordinary people do not. When a famous Hollywood actor is caught using drugs, they get sent to a luxury rehab clinic. When some poor man living in a trailer park is caught using drugs, he goes to prison. When Timothy Geitner, former U.S. Secretary of the Treasury, was found not to have paid his taxes for several years, he was forced to promise not to do it again. If you or I did that, we'd be in jail. Etc.
My take: I'd talk to a lawyer and generally be very careful.