The Glock 20 is a fine pistol, I have one and will sometimes carry it on day hikes. The 10mm round is great for self defense against man and animals.
However, I think you might want to think about the following:
First, on the Appalachian Trail you will need to conceal your gun just as much as you do at the mall. At least on the part of the trail inside national parks (such as the Smoky Mtn Natl Park, which is the part of the trail I am familiar with), you are not supposed to have a gun at all, so it will need to be hidden. You will meet people on the trail, and will probably get reported to rangers if someone sees you have a gun. The size of the G20 makes it difficult to conceal. However, I would say that I think fanny packs are the best way to conceal while on the trail, and the size of the gun is not all that important in that regard. Carrying on a belt is not feasible if your backpack has a belt on it (which it should if you have a decent pack). Coming up with a good combo for carrying concealed while still keeping it easily accessible while carrying a back pack takes some thought.
Second, I have run into many more weird/strange/dangerous looking people on the Trail than dangerous animals. Seeing bears at all is uncommon. I think we tend to over-prepare for bears and underprepare for people. I think the 10mm, while an excellent round, is more than you really need.
Finally, don't forget about weight. The G20 is heavy, and a full mag of ammo is heavy. It doesn't seem like much on day 1, but wait til day 3 or 4. When you compare the weight of the G20 to the likelihood of needing it, the ratio is pretty bad. You will find out very quickly whats worth carrying and what isnt.
Take all this with a grain of salt. Its just my personal opinion from hiking the trail in the Great Smoky Mtns. I favor the Glock 19 for this purpose. Its lighter, smaller, and easier to conceal. It has plenty of power and capacity against the type of predators I am most likely to encounter (ie, man).
I would make this disclaimer though. If you ever read the headline "Lone Gunman Eaten by Bear on Appalachian Trail", and read on to find that my glock 19 was at slide-lock, you can better believe my last thought was "I shoulda brought the Glock 20".