This is an interesting question.
I've had this theory for years that runners from D3 schools would be more likely to keep running after school as they might be in it more for the "joy" of running, and more of them would likely feel like they didn't peak in college compared to D1 schools (I definitely feel into this category with a lot of "what if" questions after college). But in my experience from my mid-upper tier D3 school, the vast majority of guys don't keep running much. A lot of guys on my team from my era (1999-2003) ran a bit over the years, and many have done other things (racing bikes, triathlons, biking across the US, etc.), but very few of them have kept up any consistent running and few are still trying to do much amateur racing. We had a couple of guys racing as professional triathletes for 5 years after, but a lot of guys seem to just not have gotten bit with the running the bug long term.
Makes the alumni run on reunion weekend a little bit of a downer.....
But I will say that way more guys run post collegially compare to swimmers who still swim post collegially (and I can't blame them, staring at the black line of a pool for hours a week gets pretty old fast).
And if you look at it from a different angle, very few of the older guys I know do easy runs with ran in college (or even high school), a lot of them got into it later in life. As someone posted above the time drain can be rough as a middle age adult with kids. I'm glad I have a wife who knows how important it is to me, so is willing to cut me some slack and let me run more than most would. That and the jogging stroller is a life saver.