I agree with Mags. It took me three weeks to get to Gatlinburg on the AT but only 2 week to hike the JMT the following year. I was a complete newbie before the AT and would say terrain had little effect when comparing my mileage between the trails. What did? Inexperience in preventing blisters(two shorter days), a couple shorter days due to lightning, hail etc, lost hiking time(which I don't regret) hanging around talking to other hikers at lunch, snack or dinner breaks, and the fact that at its heaviest on the JMT my pack was still 5 pounds lighter than when I started off from Amicalola Falls. I also started the AT in mid to late April and enjoyed hiking through the bubble and meeting lots of different hikers so I think you could start as late as early May and avoid the heat until the flat mid Atlantic area.