Why are post-college casual runners still so fast?

Training.

In college, you have to conform to a lot of runs that might not make sense, but after learning a lot. Post colligiate running allows you to train smart, and cater to your schedule which can allow for some better running. I've run all of my PRs after college.

Healthy adult males who run 50-70 miles/week and struggle to break 18 have garbage running genetics. Simple as that.

I never ran in college but ran close to 50 flat for 400 and broke 2 for 800 off of terrible coaching, my 5K was like high 16's. I took 10 years off running completely and did a mile time trial in about 6 flat in some random Hokas I bought. I think about 4-5 weeks after that with some base mileage I did a track workout of 3x(4x400) with 30 seconds static rest between reps and 2:00 between sets - goal was hitting 1:30 consistently and really trying to stick to pace but ended up averaging closer 1:25 with the last one being 1:17.

I think part of it is being used to this stimulus at a younger age when you're developing - I was easily a better shorter distance/mid D guy but now I want to race local 5k's, but I think with about 2-3 months of consistent training I could be around 18 min 5K shape. I think a lot of people picking up running don't know how to push themselves as much and aren't familiar with their bodies and paces; they don't know their limits as well and don't have "muscle memory" of competing and running fast or at a certain pace, vs someone who did it for years.

Nowadays when I start running after a hiatus I have to force my body to slow down because it just wants to get out and go fast.

I remember commenting on reddit and saying a 4-5 hour marathon is not fast - that basically my slowest long runs were 8-9 min pace when I was getting back into shape, and all these nerds who probably never ran competitively jumped on it and said I probably never ran a 5K in my life let alone a marathon.

Yeah I've never run a marathon, I don't really care to rn, but even training for 5k's my long runs were basically a HM (12-13 miles) at slow steady pace of 9 mins. Most hobby joggers finish marathons and walk a huge part of it. Most of the population probably considers running a 5K in under 25 mins as fast, since most people can't even complete it without training for it, and when they do train, they train poorly. People who haven't run much have a very skewed perspective on training and effort.

Those 50-70 mpw 18min + hobbyjoggers don't train efficiently at all. They skip the basic speedwork, drills and strides. They're too worried about being in their target heartrate zone and gaining followers on Strava. Just run baby!

They are simply maintaining and not really improving. When you can run a 4:10 mile, if you keep a very basic baseline of mileage you can run similar fast performances for years. Only spot it does not translate to is marathons

IF you ran in college, let's presume you might have been at least a little fast and talented. I ran 1:57 for 800m several times in college. Hardly a star, but mildly quick.

In my 20s and 30s, I'd run a bit over 20 mpw, about half easy and half fairly quick. Best performances were 16:10 road 5K and 16:14 track (I'd run 16:22 XC in high school.)

None of this is particularly fast, but maybe not too shabby for low volume training. For real, I'm sure there are guys who've run under 14:00 for 5k on volume like this. That's real talent, both mental and physical.