I have done a tour this long. The mileage was reasonably high with a fair bit of climbing. I kept the pace at a moderate pace to keep from getting burnt out. I ate a fair number of carbs to replace my muscle glycogen. But I did not go over board. For a large guy, doing 70 miles with climbing burns up something like a lb of fat, if you do not eat anything extra. I lost weight on the tour, but I could afford to. Most people went out and pigged out and ended up gaining substantial amounts of weight. No matter how much you exercise, if you eat enough cheeseburgers, fries and sundaes, you can gain weight.