There's a guy who always run at marathon pace!

I knew a guy back in the 1980s. He wound up burning out after after a few years, got fat, and was no longer a runner by the mid 90s.

He's doing this for like 5 years, and he has like 31:30 for 10k and 15:05 for 5k, he's the best local

Sounds like Lydiard training. Once you're in shape, it's possible to run 5 minute miles aerobically.

Maybe he would get faster if he switched some of those 3:30 kms for longer slower runs. But maybe he's having fun and doesn't care. Good on him.

...certain 'supplements' could also explain this.

Burn out is a real thing. I trained perfectly in college and
never had injuries once I got with the correct coach. I was able to run 1:50
800m, 4:00 mile, 29:00 10,000m. I typically ran 60-80 Miles a week. In 2006 I went
to Arizona, for the summer and did not have the actual local track. My training
went from perfect to erratic. In 2005 I probably could have run a 2:13 Marathon
by 2006 I was training way too much in the heat around 110 Miles a week. I did
a 10,000m time trial on an uneven dirt track that was in the process of being
dug up in 110-degree heat in 31:55 which was far slower than I usually ran them,
but the head almost had heat stroke out. By 2007 I still trained but hated it.
I did it because I was afraid of letting it go, not because I wanted to do it
any longer. There was nothing enjoyable about sitting around all day waiting
until it got cool out to go outside and do 10-25 miles.

There's a local runner here in the UK and he does his regular runs around 6:50 to 7:00, for 50 to 60 mpw but he has recently run a 3:59 mile, a 3:38 1500 and something like 13:45 in the 5000.

great times!

I really believe he can improve yeah, he seems a little bit stagnated.. but he doesn't care and keep doing easy days as fast as he enjoys

Why do you care? It works well enough for him and apparently he doesn’t give a **** what you or anyone else thinks

It’s funny how he is better than you but you know what he is doing wrong. Try worrying about your training and maybe you’ll beat him.

I didn't write this post with hate intention, im just trying to see why it works, and maybe if i can do it aswell because im not improving with the methodology i used for me, just that, if it works for him im happy

Steve Jones used to do this 40 years ago. Right out the door and into 5 minute miles asap. He didn't have time to be plodding around.