I've been running (well, fast jogging) 10-20 miles/week for over 10 years. I've had so many feet and knee problems over the years and ended up getting $250 custom-molded graphite orthotics. Those essentially acted as crutches as my arches, which made my feet very weak. Walking around barefoot caused my feet to collapse inwards, causing more pain.

Tried biking instead, but indoor biking is the most mind-numbing activity ever, and outdoor biking proved to be equally damaging (lots of falls) and far scarier (going downhill on a road with cars).

Basically I want an outdoor elliptical machine that moves at a reasonable 5-8mph.