Sunday was the first nice weekend day in quite a while, so my wife and I went running. She'd run a few times over the winter, but I hadn't been running since December. We were doing our normal 4-mile loop, but my knees started killing me after mile 3, so I walked the rest of the way. 2 days later, knees feel fine. Was this caused by not incrementally working up to the 4 miles? Am I going to hurt myself permanently if I continue running?