This is what I only run on trails. Got a knee injury when I was 25 from over use--started using an orthotic and that really saved me. And then when I was in my early 30s I switched to trail running only. It makes a huge difference. Your foot never hits the trail the same way twice so there isn't so much repetition. I tore my hamstring a few years ago and had to really rehab it. I can now run on all sort of surfaces and up/down hill. But I have to stop every 10-15 minutes to give it a break for a minute and then get going again. I found that strengthening your hips actually has the biggest impact on your knees. I have worked very closely with a physical therapist the last few years. And I do run only 1-2 days a week. The other days are swimming and biking--that helps a great deal as well. Good luck!!