If you were to hike the entire Appalachian Trail you would hike 2,190 miles which is nearly impossible but if you did somehow eventually were able to hike the entire Appalachian Trail you would burn off 219,000 calories which is a lot of calories.
You'd lose a lot of weight especially if you were a fat overweight person and was able to hike the entire Appalachian Trail.
The entire Appalachian Trail is 2,190 miles in length which is at least a 4 to 5 day drive in a vehicle for that distance but would likely take you a year of hiking to complete the hiking on the trail.
But if you hike 10 to 20 miles on the Appalachian Trail you could burn off 1,000 calories to 2,000 calories and the more you hike the more calories you'll burn off.