I decided to start running daily about a month ago, after spending years not even jogging a single time and hardly walking much at all. Didn't think much of it, just figured I'd get tired quickly since I was out of shape and I would improve over time. Managed to fuck up my knee within a week, and now I need to stay off it as it very slowly gets better. Whatever you start doing, remember to ease into it and warm up. Apparently tendons don't stay as hardy as they were when you were a kid.