11:46 a.m. on February 19, 2007 (EST)
Re: My dream is to thru-hike the AT...
There are a wide variety of styles for through-hiking. Most tend toward the light to ultralight style. Carrying a heavy pack slows you down considerably, maybe even into the under-10 miles a day range (plus rest days, which everyone takes along the way). An ultralight pack can put you into the 25-30 mile per day range.
The guru of ultralight is Ray Jardine. Even if you want to go in super-luxo style, you should take a look at his book Beyond Backpacking. You might not want to go all the way to his 10-pound pack, but you can get a lot of hints on how to lighten the load.
For anything over 3 or 4 days on the trail, I tend to shift toward the ultralight category, simply because I hate to carry loads. 'Course, "ultralight" is a relative thing. Last month, I crossed paths with Jardine and his wife in Antarctica. They were hauling sleds and into the 50-60 pound range - but you have to carry 30 or more days of food and a lot warmer clothes than you would on the AT (or JMT or PCT, or ...).
If you do the whole AT , or a major portion in one push, you will be re-supplying - even at 30 miles per day, you are talking about a couple months of food, which is in the hundred pound category. If you take advantage of the closer spacing of re-supply points, you can stay under 30 pounds total weight easily, even with a fairly luxurious sleeping and clothing arrangement. The AT, unlike the PCT, has water refill frequently enough that you can get by with 2 one liter bottles (or a small Camelbak and 1 liter bottle), so you won't have stretches where you have to carry 3 or 4 liters of water (10+ pounds just in water).
As for external vs internal frame packs, your Jansport is not a particularly comfortable pack. There are better ones that are lighter and will carry a heavier load more comfortably than your Jansport or any internal frame pack. Internals are intended for places where you need maneuverability, like climbing or skiing. On the AT, you are on good trails the whole way. Externals are cooler against your back than internals, which is important in the AT from the southern end up through at least Maryland. Most important, though, is proper fitting of the pack, whether external or internal (sounds from your post that your Jansport wasn't fitted properly). Get together with an experienced packfitter in your area. A properly fit pack makes all the difference in the world, second only to properly fitted boots (most through-hikers these days, especially the ultralight crowd, use trail-runners rather than boots - what I use for anything over 25-30 miles, unless I am carrying a really heavy load of climbing gear).
Before you head out on your thru-hike, go through your gear very thoroughly. Lose anything you don't really need and replace what yo are taking with lighter gear that will serve the same purpose. You can probably cut your sleeping bag weight to 2 pounds or less and your tent to under 3 pounds. Use a closed cell foam pad instead of an inflatable (less than a half-pound vs close to 2 pounds). Consider the weight of your cookpots carefully (you probably only need one pot, for example), and the weight of your stove. Do a trade on the type of fuel you will carry (for long trips, white gas stove plus fuel weights usually ends up lighter than compressed gas when you include the empty canisters that have to be carried out). Consider how much clothing you really need (all the other thru-hikers are skipping baths, too, and no one cares about your clothing styles out there - plus you can cache changes of clothing at some of your re-supply points, which you will want to do for socks in any case).
Planning and lots of thought is the secret to success.