I have the same problem.
Went from running 33 10K and 2:36 marathon and then poof, literally overnight I cannot break 40 minutes for a 10K
My right leg just limps along. Some days are better than others, but yes, usually on the roads, after around 4 miles or so I gimp like a cripple.
I have about a half inch leg length discrepancy. I wore an orthotic on just my left shoe to compensate.
I never get hurt. Hammies get tight some, but I have not been hurt for a year now.
Just cannot run fast anymore. I am old now, 41, but this problem started for me when I was in my low 20s.
I have the worst flexibility you could imagine. Perhaps that is the problem.
I did 15 miles yesterday, a long run for me recently. I was fine for the first 13 miles, then I limped considerably the last 2. Very frustrating since it was basically an easy run too.
I prefer to run trail races now. The surface is so screwed up on those trails the limping is not a factor. Plus I found out that if I walk for maybe 20 seconds or so, the leg kinda recovers. Which means your 10K days are over, but you might still be able to run some longer races.
Actually I am 41 years old now. I think I would like to break 35 for 10K this year. But I realize to do that I would need to walk a few times in the race. Not because I am tired, but because I would start to limp along.
Right now I am running in Nike Frees....they feel nice. It does not matter if I run in boots, still at a sustained fast pace I will limp after 4-9 miles.
I am glad to see this post. I have this problem. I was too embarrassed to post this problem. It is embarrassing and extremely embarrassing.