It would really be nice if this was a skill that could be improved upon via experience. I know that there's a feat that can be unlocked by sprinting a lot currently, but it would be neat to see a mechanic that could be considered Physical Conditioning; it only makes sense that living in harsh conditions and surviving should make you stronger physically, much like weight lifting or working out. After carrying so much stuff around or sprinting everywhere, perhaps your body adapts and you eventually are able to carry more weight or tire out slower.