As everyone says, walking gets you in shape for walking, and gradual increases in intensity are the way to go for long term health and fitness. One more thing, once you get to where you have been walking consistently for a while, is to get in all the uphills you can. Goign uphill you'll get a much more intense cardio workout than on the lfats, and whn you are on the AT, uphills will be the standard fare.
To take it easy on the knees, I like to find a route that allows me to go up a steep hill and down a gentler one.