Opinions vary. It sounds like you're interested in running to stay in cardiovascular shape more than fat loss. If that's the case, then 3 X a week is good. You probably should hold your heart rate elevated for at least 15 minutes (20 might be even better) to get a cardiovascular training effect. Also, if the pounding on the treadmill is hurting your joints then switch to a glider, stair stepper, jump rope, or do some light heavy bag work. Any more than 3Xthat and the cardiovascular returns begin to diminish. If you were cutting up, then I'd recommend a combination of long duration low intensity in the morning on an empty stomach (ie. brisk walking for 30-40 minutes) 5-7 days a week and then hard interval cardio (jump rope, heavy bag work, sprints etc...) 3 times a week for 20-30 minutes or so. Many will tell you that running will interfere with your strength training. There's some truth to that, but in moderation like you're doing it is very good for cardio benefit to stay healthy, control body fat, and even workout those stiff muscles and joints. In moderation you'll probably not notice any appreciable reduction in gains. Also, I find that having a strong heart and lungs and enables me to train harder with weights.
