Get some maps of the trail in the areas you plan to hike - maps that include elevation profiles. Those profiles will be very educational with respect to the big picture and the day-to-day challenge. What they won't show you is the difference between a flat place in Pennsylvania (covered with hundreds of stumbly rocks) or a flat place in Maine (roots, rocks, more roots, more rocks) or a flat place in Vermont (mudhole, or maybe a boardwalk) or a flat place in TN a bit south of Damascus VA (smooth, easy, sailing along). All that stuff, you get to discover when you get out there.

The other thing the profile maps won't show are the smaller ups and downs that don't get picked up by whatever criteria the profile map is built on. If the profile map picks up 50' changes in elevation between point A and point B, it won't show you whether the trail goes up and down 40' repeatedly between A and B.