Jekyll Island to Fort Wilderness shows a 3.5-4.0 hour drive per Google Maps.
I for one would NOT tow a camper or drive a Class A down I4 given the construction from mm95 to about mm74. (I was in this area between Christmas and New Years) I would take the Hwy 417 bypass around (it's a toll road but worth it) starting at mm101b on I-4, exit onto Hwy 536 and follow that straight into WDW which becomes Epcot Center Dr (which is clear of race/marathon stuff by about 9am). Then it's a short turn off onto Bonnet Creek Parkway and on to the Fort.
Here is a pic of getting off 417 (bottom right), riding in on Epcot Center Dr, zig zagging down onto Bonnet Creek, then on to the Fort:
View attachment 375137
It costs less (fewer tolls) if you take I95 south past Daytona (skip I4 altogether) to Titusville then take Hwy 528 west toward the Orlando airport and pick up Hwy 417 and into WDW the same way as I previously described. But it's a few more miles out of your way.
I really think the I4 to Hwy 417 is the way to go (it's still fresh in my mind). I did this route only two weeks ago.
Bama Ed
PS - the marathon runners start filling up Epcot Center Blvd from about Buena Vista Drive around to the front of the Epcot parking lots starting at about 4am (been there, done that). It's a lot of standing around and waiting. At 530am they start releasing the runners in bunches every couple minutes in "waves". By 7am all the runners have started and Disney is quick to clear the road of fences and such to get the road open for regular park goers. So it should be fine by the time you arrive.
By the time you arrive, runners are straggling through Animal Kingdom, trudging over to Wide World of Sports along Osceola Parkway, limping up World Drive for a brief trip through Hollywood Studios, and then celebrating along the canal into Epcot via the International Gateway, the countries, out Epcot under Spaceship Earth, and the delirious finish line in the Epcot parking lot. (been there, done that). These places are not along your route so they should be out of your way as you come into the property.