By the end of the 1st mile I was hurting. Soon I was limping, by halfway point hobbling. Refused to quit, took almost 2 hours to complete, and my knee was swollen like a watermelon. It was months before it healed enough to do better than a hobble, and it's never been quite the same since.
The sick part is that I didn't learn from this -- about 10 years later I injured my shoulder while kayaking and wouldn't quit kayaking even though it hurt, just kept taking ibuprofen. Finally developed a serious bursitis that took me out for months. Couldn't wear pullover shirts 'cuz I couldn't lift my arm high enough to take them off....
Quitting 'cuz you're tired is one thing but when your injured sometimes it's the right move.... :)