In reply to hopskin316:
fell runnings the shit. a great thing to get into.
make the effort to get good shoes and try on a few choices. crosslites are great and nimble (you can tell LA make climbing shoes) but also pretty minimal.
if you want more structure to the shoe have a looksee at innov8s range.
go easy on yourself until you work out a good technique for descending at pace - that may be whats beating your feet up.
if it helps, a shoe with a toe box you can really push into (ie not too tapered or tight, and well laced) so you can shift to a toe strike on the way down might help.
also, if you see any of the skyracing stuff, they use a technique where they drop down sideways rather than facing into the path, as a way of variating the muscular impact.
training youself to bend further into the strike part of the stride, and to lengthen your stride with a longer, more streamlined movement that uses more upper body and arm momentum spreads any impact out rather than running like a robot.
dont forget to use your arms as extra lift on ascents.
single legged pistol squats and lifts help with that, as well as the core strengthening to tie it all together.