I would say mental state is a big part. One can push thought. Physical limitations will take over. Depends on how many days for myself. Retired Military and I can still push thought, but now the physical limitations will take over after a couple of days. I have gotten to the point where when back three miles in the rough, do I really want to shoot this deer and have to drag it out? Helps if you have younger family or a guide that expects to give some deference to the "old man"