I've been running about 8-10 miles/week for 15 years now, but have never been able to expand beyond that very much without excessive knee pain, foot pain, and m

I've been running about 8-10 miles/week for 15 years now, but have never been able to expand beyond that very much without excessive knee pain, foot pain, and muscle loss (I weightlift a lot too). There was a period of about a decade in which I had to wear custom-molded graphite orthotics (~$300) during most of my waking hours because my feet were supposedly "flat". Two podiatrists said that I'd need to wear them the rest of my life or get surgery, which would leave me in crutches for 6 months per foot (they won't do both at the same time). I lost my orthotics in a flood, and unwilling to fork over another $300, I focused on fixing my feet. I started stretching my soul, walking around the house barefoot, and going for short (~100m) barefoot jogs on a grassy field. After a few months, I started wearing shoes without arch supports to force my feet to adapt. Managed to get rid of my foot pain within 2 months :). Still struggling with the knee issue though. I've been treating running as an isolation workout on the knee joint -- my body can handle it just fine, but like in weightlifting, I can't isolate the same muscles/joints day after day. Sprinting any amount hurts, and I have to avoid all hills (going up is fine, the descent is killer). While I figure out a way around the knee pain, I've recently been doing more metabolic conditioning (think Insanity or P90-X, but with fewer knee-destroying jumps). I experience a lot more fat loss this way, plus the workouts seem more holistic than running in the sense that my upper body is not neglected.