These aren't really big treks, but they will get your heart pumping. Plan these at end of July to mid August. There will be snow at the higher passes - just follow the foot prints of those before you.
Lots of very nice 40-200 mile hikes in the Sierra. Well, ok, the JMT and PCT too. Entry from the east side from US 395. Trail heads start at 9,500+ to 10,500'. Get to lakes in under 6 miles and a pass the next/same day of around 11-12,000' down to more lakes. These 'feeder' trails connect along the Pacific Crest Trail (PCT) or John Muir Trail (JMT) to an exit of your choosing.
There are a few loops such as from/to Onion Valley out of Independence. Catch a thumb ride down to road to Symmes Creek 6,000' start of Shepherd Pass trail (you will have to cut and paste the links).
(and a couple of 14ers at the top of the pass) down Tyndall Creek to the upper Kern River basin for as many days cross country exploring at 10,500+' you might be interested in. Then over Forester Pass (13,200')
to Onion Valley to pick up car. Its a 4 day strenuous trip but can be extended into all kinds of places to explore. Shepherd Pass trail is express ride to the high Sierra. The area between Wallace and Tyndall Creeks has a lot of opportunities to just enjoy being there.
(Wright Lakes are just south of Shepherd Pass - Look NW for South America Lake and a sharp ridge that has a series of passes that look into another major drainage)
Loads of stuff to do on the CDT (Continental Divide Trail) eg:, Follow Pine Creek down from US24 (North east on map), Catch Mt Harvard before you go over Elkhead Pass, catch Missouri, Oxford, Belford, before exiting down to Vicksburg, hitch a ride back to car. You can make this a 4 or 5 day trek.
Well,if you plan to start the AT in March and are looking for a big trek starting in January in Florida why not do the entire Eastern Continental Trail which starts in Key West and ends in Canada and connects the Florida,Pinhoti,Appalachian and International Appalachian Trails all in one long super trail.Can't help you with Fl hiking partners but Sandra Friend at Floridahikes keeps track of the 2016 Florida trail thruhikers class and may have knowledge of potential partners.