My experience doesn't exactly mirro Evan's all the time. There are longer hunts that are go-go-go from the time you jump out of bed to the time you crawl back in. But really that is rarely day in/out. Sure you will bust your butt getting your camp and what not in to where you hunt, but once you are in your hunting locale you shouldn't be hiking non-stop all day long. In fact once you get in the hunting area, SLOW DOWN. Glass early and often. Sit back and glass and enjoy just being out. Backcountry hunting can be as grueling as you want it. However, often times the more time I spend behind that glass and less time I spend hiking the more successful I am.
As far as getting hungry?? I don't. I bring enough food and there are times I am not hungry but I eat anyways knowing I am burning a lot more calories in teh backcountry when logging some serious miles or gaining some elevation that will require a LOT more energy than what my body uses during an an average day in the off season where I may only train for an hour of two vs. 12 hours of hiking/packing. Just bring as much food to cover your needs (I plan on 1.5 pounds per day) drink plenty of water, have a quality pack, and sleep system. Then go enjoy the backcountry for as long as you want so long as you got the food to do so. Really for me the only difference between a overnighter and a 10 dayer is the amount of food. I don't really feel as if I am falling off my game after a couple of days, maybe some do, but I haven't experienced that, well atleast that I've noticed.
You've already done 2-3 day trips so its just more food as I stated before to go longer. Have fun with it and don't try to make it harder than it has to be. Look forward to hearing about your longer endeavors on here.