We drive. For us it's about a 7 1/2 hour drive down to Orlando. But I'm with Keith on this one. If it's 15 1/2 hours, I'd make a 2 or 3 day trip out of it. Maybe drive down to Nashville or the Great Smokey Mountains. Perhaps spend a day in Gatlinburg or Cherokee, NC. Then drive on down through Georgia down to Florida.
When my DW and I took our first Disney Cruise, even though it's only an days drive for us, we took our time with it. Drove the first day down to St. Augustine, FL (about 5 hours). Stayed there a day or two. Then, the day before the cruise, took a leisurely drive down A1A to Port Canaveral. It was probably the most enjoyable vacation we've ever taken!
Tom (... slow down and smell the roses!)