Multiple "long distance" trails?

I'm more of a lurker here and a meticulous planner for the next year or two before I can actually get out on the AT. I've still got a year on my contract before I can get out of the Navy so I'm trying to do all my planning now so that I can stick my foot out the door the minute I get my dd214 in hand.

I expect to go to college immediately after I get out and hike the AT, however I'll be getting out in fall of '14 which leaves me with almost 6 months before I reasonably expect to be on the trail. I'd like to do some distance hikes before I actually get on the trail. Which leaves me to question of how many of you guys have done more than one of the major trails in the states? I was thinking of the Florida Trail for a warmup, but thats not much shorter than the AT itself (although Florida is sooo devoid of any sea level changes).

Also while in college I was thinking it'd be neat to pick a trail and hike it over summer break, has anyone else attempted something like this?

For summer vacation hikes, the John Muir Trail is a beautiful two or three week hike. Colorado Trail is six to eight weeks or so. The Great Divide Trail in Canada is more difficult, but very beautiful and good for 6-8 weeks. The Long Trail in Vermont is also a good long challenge. Pennsylvania and New York both have some good long trails like the Finger Lakes, MidState Trail, Donut Hole, etc. You can link together long trails there to make a trek of any distance you choose.

In the Autumn you could do part of the AT until winter stops you, or hike a couple of the eastern trails I talked about above for a couple of months before heading south to Florida, or go to the Ozarks, or head west to the Arizona Trail or the Grand Enchantment Trail. The issue of hiking in winter is more difficult. The Florida Trail is usually started around January (I think few do it earlier because of hunting season.) Most of the long desert trails out west go up into higher elevations where there would be snow. You could head to New Zealand or Australia -- or just go for a wander around South America.

Spirit Walker pretty much covered it.
For winter hiking, my choices would be: anywhere on the AT in the south. (of Mason Dixon line), Florida trail (although even that can get too cold, ie. 2 years ago), Peru, Copper canyon (Mexico), Joshua Tree NP (CA) or Anaz Borrega desert (southern CA).
But, remember Sept is normally the best hiking season almost anywhere in the northern hemisphere (OK south of 50 deg.) and Oct is great too as long as you don't mind some early season snow. (which would be good practice for your thru-hike anyway)
I don't really understand when your schooling is going to start. Or where.
Good luck and Enjoy!