The math seems fishy here. Let's assume your young woman's base metabolism (without any extra exercise) is ~1500 calories a day. Running 5 miles a day burns 500

The math seems fishy here. Let's assume your young woman's base metabolism (without any extra exercise) is ~1500 calories a day. Running 5 miles a day burns 500 calories putting her at 2000 total. For her to maintain 2000/day while running 15 miles a day, her base metabolism would've had to drop 66% down to 500 calories a day. I simply don't believe that. Your metabolism might drop a few percentage points per year, but the drops will eventually tail off as your metabolism reaches its lower limits. You can't just linearly extrapolate the drop down to zero, as your examples imply.