Hard court tennis is a bit harder on your body than clay, but it's not a whole other sport. Tennis is still tennis, running is running, 4 hours of running back & forth, back & forth, is still tough, on any surface. The surface difference makes it maybe 20% easier at most. Either you can do it or you can't.
For someone with a bad knee I think hardcourt tennis is a huge difference. Hardcourt is really tough on your knees and ankles, especially the way these guys run and slide on it nowadays. I could understand it if Nadal decides to play fewer hc tournaments. Would probably be one of his best decisions in his career.