It's more or less part of my career to keep fit, so it's good that I don't worry too much about it. For me these days I try to go for a run daily, of about 5km. Some days due to half marathon training I will cover long distances, but will need rest my knees in between the runs. I don't actually enjoy running a lot, but I'm motivated through the Nike+ which plugs into my nano when I run and keeps me updated with my stats.