How many miles a day? At 5 mpd, that looks about calorie-neutral (no weight gain or loss, or, if so, weight gain from burning a bit of fat while putting on some muscle).
But if you’re doing 15 miles a day, up and down trails, you’re about 1000 calories short, depending on your weight. Maybe that’s fine – burn a 1/4 pound of fat each day, and you’ll come back 3 pounds lighter and leaner – something most of us could benefit from. But not everyone enjoys it if your reaction to that calorie deficient is to constantly feel hungry. Manfred budgets burning 1/2 pound a day(!) and drops 15 pounds on a 30-day trip, but (in so many ways), most of us aren’t Manfred.
And it gets harder to correct on a high-mileage trip. At 5-10 miles a day, you browse on more berries, rose hips, etc, as you go. 20+ miles/day and you’re using every bit of Fall light to get to your next camp.