The short of it is, I've got a 5K run coming up in about 5 weeks, and no training. I need to shore up my worst weaknesses just to complete it, but how?

In more detail, I'm 53 years old, in pretty good cycling shape. I ran the shorter distances in track in HS, and then basically nothing until a half-assed 5K about 4 years ago. I did a little running Sunday and today - the run-walk thing - and it's even worse now. Knees are weak (I didn't expect that), groin tightens up, and balance and coordination aren't that great either. I figure the most urgent is strengthening the knees without injury. What's the best method in the time available. Would stairs be viable? Some weight training? Or would you tough it out and just run?