I stopped running for the summer and switch to cycling and swimming to give my joints a rest. When I started again this fall, the first run back sucked ass. Everything hurt, I couldn't get enough air, and my pace was embarrassing (compared to past performance). But just one week later, my legs toughened up, I felt bouncy, and my time improved. It's amazing how quickly the human body can reconfigure itself to meet a challenge.
Runs are back to feeling fun.