This reminds me of some friends I went with one year. This couple friends of mine were both HEAVY smokers and drinkers and very out of shape, but not real over weight just about 30lbs or so. After the first day of a single park they were both spent and acted the rest of the trip like they were 90 years old, stayed in the hotel most of the rest of the time. Sounds like you are already in great shape if you do a mile or 2 a day. I probably should do more walking before my trips for they do wear me out, but I run on adrenalin so I keep going strong the whole time despite the fact that I will get tired about half way into it.