1/2 marathon training

So I got this crazy idea to sign up for a half marathon.  I have been training for the last 3 weeks, and because the knee issue it has been all mostly walking with some biking, yoga, and zumba to help cross train.  I have found myself liking yoga more and more each time I go.  Today was my 7 mile walk.  So far everything seems to be ok, my legs started to tighten up on the last few miles but a middle of walk stretch helped that. The 1/2 is Sept. 7th and its a Freedom Run for 9/11, and the first responders.  So I will be up against people who have to run for a living, I am not expecting to beat anyone except myself. I am doing it to prove to myself that I can, and will do it.  I don't care if I am the last one who crosses that finish line as long as I cross it.