On the Trail
At the beginning of your thru-hike, start out with a goal of about 8 miles a day. Gradually increase distance to avoid injury and enable your body to adjust to the rigors of carrying a full pack all day on rugged terrain. Plan a "zero" (a zero-mileage day) in town, or at least a "nero" (nearly zero miles) occasionally to give your body a chance to rest.
Allow several weeks on the Trail to get into peak condition. Younger hikers may need less; older hikers more. Knee and foot injuries, stress fractures, and shin splints force many hikers off the Trail—the risk of these can be minimized by keeping your pack light and your mileage conservative in the beginning.
In very general terms, the terrain of the A.T. is most challenging at the ends and easiest in the middle, although Maine and the northern half of New Hampshire are considerably more rugged than any other part. Northbound thru-hikers do their biggest miles from Shenandoah National Park in Virginia through Vermont; longer daylight hours also allow for bigger miles.
A very useful analysis of thru-hiker average mileage through different sections of the A.T. has been compiled by "Map Man" and is available on the WhiteBlaze.net web site here.