I'm about your age and planning to complete an AT section hike this summer. I have about 250 miles to go in New Hampshire and Maine. As difficult as this region is, I feel like it is unwise to head out there without improving my endurance. Even with a resupply, there appears to be a certain minimum of conditioning to go through the Hundred Mile Wilderness in a single trip--being able to average 10 miles a day with a pack for 10 days. If I had to start the trip today, I'm probably just above that minimum.
My guess is that, if they have sound muscles and bones, and aren't severely obese, a lot of people do hike themselves into shape on the Northbound AT from Georgia. The chances of doing that Southbound are slim to none.