my knees are messed up too, but they can hang in there if i up my mileage slowly over time and stay in shape... and if i don't run too fast or down hills.
Last spring I got really serious about beating my longest distance... previously it was probably 6.5 miles or so.
by the end of the summer I was able to run ~ 10.5 miles. It was a new experience for me because in the past I've been so limited by my knees that I never REALLY felt the rest of my body get sore and ache from runs!
This spring I got back into running (i was run out and took a break between November and March). It was a slow start, but I'm back into the swing of things. I've already beat my distance from last year.
Twice so far I've run 13 miles (the second time I thought I ran farther, but when i drove it to check the distance it was exactly 13 miles... I was pissed). Each time though I had to not run for about 5 days after.
I plan to keep going for that distance until I can do it and some light runs a little sooner after. Then I might go for 15... I'd love to get to 15 by then end of the summer, but even though its just a few more miles it seems impossible right now.
We'll see.