Find something else that makes you happy, don't go around agonizing over your time. Whats going to change if you go sub 3?? My PR is 2:49 and I once thought I could spend a few years training hard to go sub 2:30 which was my ultimate dream. But I didn't even try. Why? Because nothing would change in my daily life and my family life, QOL, and job performance would likely suffer. Now I run for fun and health and am happy running a 3:30-3:45 marathon each year.
With that logic there's no point in trying to be as good as you can at any hobby. Maybe the line is when the cost of improvement get too high idk.
I am surprised that only two or three replies have anything to do with how to break three hours. They are mostly inside jokes about someone named Greg? Is that a thing? Who cares about Greg? Weirdly, it feels like everyone on this thread is referring to shared knowledge that the rest of humanity has never heard of (nor cares about).
As for breaking three, here is an honest reply:
1) how old are you? How many marathons have you run? What were your times (do you see progression)?
2) what is your body type? Are you a strength based athlete or lean/lanky? Do you have a lot of extra body weight (like I do) that could be dropped?
3) what kind of mileage can you sustain without getting injured?
4) what are your times at shorter distances like the 10km and HM?
5) What kind of long runs have you traditionally done in each build up? Four over 22 miles? Or one at 20 miles?