usman88 said:
... I've heard that biking long distances (70-100 miles a day like we're doing for 70 days) tends to burn more carbohydrates and less fat...!
Actually it's the other way around. Long rides like you describe burn an awful lot of glycogen(carbs) but unless the intensity is very high you'll burn even more fat.
The ratio of glycogen to fat burning goes up with exercise intensity and typically goes down with exercise duration. So all day rides, especially if you're touring loaded will burn a higher percentage of fat. You'll still need to replenish the glycogen(carbs) burned each day if you want to be able to continue the next day so you're going to have to eat quite a bit each day just to keep going from day to day. You should still burn off an awful lot of fat on a ultra-marathon like that.
Check out a calories burned estimator like:
http://www.nutristrategy.com/fitness/cycling.htm and consider both your current weight and the pace you think you'll realistically hold for those 70 to 100 mile days. Those are estimates of total calories burned but you should expect at least half and probably more like 70% of them to come from fat for all day rides on a multiday tour. So you should plan on replacing most of those calories each day, being especially careful to consume some high carb and ideally high GI foods in the first half hour after you stop cycling every day. That critical half hour is essential for quickly replenishing glycogen stores which will help you the following day.
Even with a plan like that you should expect to lose weight since a typical steady 100 mile day of cycling will burn something like 4000 Calories depending on your weight and pace. Add to that the 1500 to 2500 Calories of Resting Metabolic Rate and you're going through an awful lot each day. It's pretty darn difficult to consume more than 4000 Calories a day on a regular basis (I know I've done it for over three weeks at a time on climbing expeditions to places like Denali and it's hard to make yourself consume so much day in and day out) so you'll almost certainly be running a caloric defecit and losing weight.
Dropping a few pounds between now and the start of the ride will really help you enjoy the tour, but this is one I wouldn't plan on starting whippet thin. You'll need a few extra stored Calories to go the distance to Alaska.
Good luck and let us know how it goes,
Dave
