I stopped running after college track and about 2 years later I hopped into a 5k in my hometown just for fun and ran 19 minutes. To put some of this in perspective, I didn't do ANY running, lifting, or other sports for 2 years. My best 5k I ever ran was a 17:30, but I wasn't really a distance runner. It's been about 5 years now and I think I might finally be at that point where I couldn't break 20. Obviously getting back into shape would change things and if I stayed in shape other ways like lifting or even joining a kickball league I'm sure I could've gone longer. A guy I went to high school with came with me to do that 5k I mentioned earlier. He ran 16 minutes back in high school, didn't run in college, hadn't done any physical activities since we graduated, and would occasionally smoke cigarettes on the weekends at bars and parties. He ran 17 minutes that day after taking a 7 year hiatus. I'm sure he could still break 20. With that said, I'm sure most talented distance runners could go 15+ years and still do it.