The max I ever did was 12 milers.It's so annoying as I really enjoy it. Sacrilege on here to say this, but it gave me a better buzz than cycling.
The real buzz I got was on the home straight, maybe last 1/4 mile after an 8 miler, when I still had it to accelerate. My running mate (now my cycling mate) was a better runner than me overall but I could pip him on that home straight.
Ahhh if only he Mo.