I miss running

It has now been three weeks since my last run. All I have been doing is swimming, ab / core work, and physical therapy for my right knee. On the day of my last run my knee felt sore, especially when walking down the stairs, but when I got to the gym, I thought I'd give a few minutes of running a try. I felt fine while running, so I continued to run for half an hour, and then I jumped on a bike for another half an hour. When I began to head down the stairs to leave, I could barely handle the pain of each right step. So, for several days all I did was rest and ice. Eventually I was given some physical therapy exercises from some friends, and now I'm spending as much time as I can doing those. As of today, I feel fine going up and down the stairs here at home, but I'm scared to run and have the pain all come back. I'm not sure when to try running again. I'll probably start by seeing how some biking goes next week. My concern at the moment is being ready for the only race I've actually already signed up to do this year - the Cleveland 1/2 which is only 13 weeks away now. I kind of wish I hadn't been so eager to save money and sign up so early. Hopefully I can start training again soon and be ready to race!