You should never expect to hit the trail "running".
If you're a young athlete there are still knee issues. Georgia and Maine are two very challenging states on the Appalachian Trail. Heading north or south, its easy to blow out a knee.
Hiking on the Pacific Crest Trail is considered easier because some of it is "graded" for horses. You'll find this true in the first sections, and because of water concerns, twenty mile days will happen pretty early on.
If you're on a trail which requires much navigation (not well marked like the AT) you will require more time and hence achieve less miles in difficult sections. When you feel lost, sit down and figure it out. Wandering around aimlessly only makes things worse. There were times in the Bob Marshall Wilderness on the Continental Divide Trail where we had to head up trail just to check a marker or cross a stream. The guide books being old and sketchy, this is a common occurance. Buying forest service maps, the Delorme Atlas, and up to date information is crutial.
Before buying a trail guide, check the publishing date.
A light pack is going to help your knees and ankles even more than your back. Visit http://trailquest.net/ultralight.html
to read my page on ultralight gear, techniques, theory, and partnering.
The first week you're on trail, ten to twelve mile days are fine. Don't worry, or let someone nag you into over extending yourself. If you wake up refreshed, with only minor aches and pains, you can up the mileage.
Averaging 100 miles a week should be very doable with in two weeks, including the resupply in town. With 100 miles a week, you can complete the Appalachian Trail in 5 months, the Colorado Trail in 5 weeks, the Pacific Crest Trail in 6 months.
Generally, a young man or woman would have greater mileage days than an older retired person. But, you'll find exceptions to this. I met a guy named Dell on the PCT. He was an ultra marathon runner whose hiking style differed so much its worth noting here. He was 60 years old.
He ate cold food, hiked a steady pace until he got sleepy. Pulled out the tarp, slept on trail for perhaps 4 hours. Then, he'd get up and go again.
Sometimes you'll meet young hikers who party all the way up the trail, and never finish. Getting "hung up" in a trail town can waste valuable time and money and end up costing you your thru hike. The movie you're dying to see will be there when you finish your thru hike, don't be side tracked.
Some portions of a long trail are "easier" than others. That's a good time to "bank " your miles, and put in some twenty mile days. By the time we reached Pennsylvania, anything less than a twenty mile day seemed wimpy. Doing twenty miles daily on the Pacific Crest was considered very reasonable, especially where towns are few and far between. The choice of hiking long, or carrying days more food is something to consider each time you're buying supplies in town.
Shorter trails, like the Bartram or Foothills in the south east are much like the Appalachian Trail in terrain. Yet, you really don't have the length to get in shape for huge miles. Be content to do 12-15 miles, and enjoy it.
When planning your itinerary, having some idea of expected mileage can help you plan post office drops, how much food to carry between towns, when your family can expect you to call in and touch base, and give you a way to plan gear swaps.
As you progress up the trail, you get into a rhythm of waking, eating, hiking, and making camp. Until you actually do it, its hard to tell what that rhythm or style might be.
When I'm solo, I rise with dawn, pack up, and while hiking greet the sun while munching a pop tart or a snickers bar. If I have a trail partner, usually we make coffee and get on trail a little later. There are different styles of long distance hiking which affect mileage.