My last foray into running was the most successful. I started the C25K program in March and I kept running until June. While I don't have knee issues, I have had other things that come up when I've run in the past.
That said, I find that running is just hard. My speed improved this time, but once I got into the part of the program where there were no walking breaks, I started losing my enthusiasm; my speed started slowing and I was always praying for the run to be over. It took me 1- 2 days to recover from running and this affected my cycling. My legs just felt dead. I wish this didn't happen, but since I am not going to let anything interfere with riding, I stopped. I just didn't have time to run, ride, and 2 boot camp classes a week, and be able to get the number of cycling miles i want. I still get some running/sprint in at boot camp and I can keep up with the real runners/tri-athletes here.
I know I made the right choice, because when I am on my bike and see runners, I'm not jealous. But, when I used to see riders when I was running, I always thought "I'd rather be on my bike."