Man those Rail/Trails are FLAT!!! The Withlacoochee is 46 miles long and not too bad, the Van Fleet is 29 miles (with only one bend for the whole length) and the combined Lake Minneola/West Orange trail is 22 miles but a bit more urban than the other 2 I've listed.
We've been riding in Central Florida for the last 5 winters and just bought a house in Clermont. We find the road riding in Lake County to be fine so I wouldn't suggest only riding on Rail/Trails. The Florida Freewheelers have a lot of nice route slips available on-line for the Central Florida area which will give you a nice range of rides. There are actually hills available in Central Florida but nothing that a California team couldn't handle.
Here is a nice link for you
http://www.trails.com/stateactivity.aspx?area=12837
Edit: added some pix of the 3 trails. The first pic is the Van Fleet, the second is the Withlacoochee and the last 2 are the West Orange Trail.