Many people think that it is relaxing and easy to quietly sit in a treestand and wait for a poor, unsuspecting deer to walk by. They are terribly wrong. You need to be in shape to sit for many hours in a tree, especially if you sit from daylight to dark during the rut. The swaying of your tree from the wind will constantly use every muscle in your body to keep balance. If you are out of shape you will lose concentration quickly.
Walking around in rough and hilly terrain likewise is demanding. Try dragging a two hundred pound deer for a long ways back to the vehicle. Long hours without sleep will furthermore drain your energy, if you are not in shape. Setting multiple stands in the spring or fall takes a person who is in good physical condition. Otherwise it can get dangerous if you tire quickly. Just climbing into your stand in the morning or back down in the evening takes good balance and strong muscles, if you are to be safe.
Summer is the time to get back in shape, if you have to. Take long walks with your spouse to increase stamina. Do some weight training. Go on that diet that we all hate. Lose those extra pounds that you do not want to carry up that tree or over that hill during the hunting season.
Here is one good excuse to go fishing. The movement of a boat in the wind and waves closely simulates the movement of a tree in the wind. This will help in building the muscles necessary to be comfortable for hours in a tree. I bet you never thought of that one before.