Hi Molly733!
My three children and I drive from Biloxi to WDW every year, and it's a very easy drive (if there's such a thing with three kids in car!!) We take I-10 East to I-75 South (409 miles), then I-75 South to Florida Turnpike(S.E.) Exit 65 (115 miles), FT to I-4 West (47 miles), then I-4 West to your WDW EXIT!!!! (about 10 miles).
Don't forget change for tolls. Last year they were $2.00 and $0.50 going and $2.50 on return.
Also when returning to the Florida Turnpike ( I missed this the first couple of years for some reason)I-4 EAST to Florida Turnpike (EXIT 31); bear right SR-91 to Turnpike NORTH.