I think the mistake you're making is thinking that former runners are all special. Most people get fat as they age. It's typical, even if it is unhealthy. Former runners are the same. Some stay trim and fit, most don't, just like the general population.
I understand why you just assume that is s a group they would be more likely to dedicate themselves to fitness, just from a selection bias. However, I think there are a couple reasons why this turns out to not be the case:
- most didnt start at 14 years old with fitness in mind, they were attracted to the competitive nature, so it really isnt any different than looking at former college football players or former college golf players
- while some may be interested in fitness as they get older and stop competing, the most obvious form of exercise is their former sport, so it is hard to transition to being a fitness jogger. Many fall into the trap of trying to run too hard while not training and either burn out on exercising or fall into a spiral of injuring themselves, so they eventually just give up
I try to caution all NCAA runners I know as they finish their senior year to learn that 2nd lesson, in the hope that they can transition successfully to fitness jogging if they're done competing.