are there exercises to keep your back from not hurting (or do u just practice for 6 hours and ignore the pain)? are there exercises that you do to make your physical condition optimal for piano playing? AND, if your fingers/arms become sore after practicing a mini-section (i.e., a part of a phrase), do you take a break from that section, let your muscles recover and then hit it again OR do you try to build up your endurance and practice through when your fingers feel rubbery??
Or work up to it. The body takes time to adjust.