Besides not committing to fitness earlier...

Taking high school too seriously and putting too much faith in my teachers/counselors knowing what was good for me. I did very well in high school, academically, and took all the high level math and science courses I could. In my senior year, I took biology, AP biology, chemistry, physics, pre-calculus, calculus, the government required English course, and AP English.

I did great, even got some university credits. But you know what? I hated it. It was tons of work all done in the name of getting into a good university so I could get a good job in the sciences after I graduated. I did this because it was what family, teachers, and school counselors told me was the good thing to do. I ended up going to university on their advice, and burned out within two years after realizing my job prospects weren't very good in the field I had chosen, especially since I sucked at "networking".

I'm now in the process of getting a job I could have applied for straight out of high school without taking on loads of debt (I would have even been in a better position to get it if I had applied then). It's what I actually wanted to do after graduating high school. And if I had committed to that instead of doing what I thought society and those around me would approve of, I could have taken the courses I wanted to in high school (and even one which would have been relevant to the job I'm looking at now.). If I could go back, I would still take some of the math and science course, but I also would have followed my passions, like history, language, and music. I probably would have been less of an uptight, asocial nerd as well.

tl;dr: I regret following the standard "good student life script". I didn't want to go to university, and I wanted to take classes in high school that were relevant to my interests, not what is considered responsible.