I am still running for exercise but not very long distances. I do a 5 mile run and a 3 mile trail run per week and I find that keeps me going. As for knees, I seem to have been blessed with knees of steel because they never bother me even on long down hill stretches.

I just get tired after 8 miles or so. I might beat that if I went on some longer hikes to acclimate better but so far 3 days is the longest I've been able to get free. Working for a living sux.