Mapquest says it's 9.91 miles and 14 minutes drive time, but I've gone from one to the other in 10 minutes or less, get on I-4 and you're right there( as long as there's no traffic !)
It's an easy drive. It took us about 20 minutes from the Hard Rock Resort to the parking lot at MK. We left MK at 1:30pm, drove back to the HRR to let the kids rest and went back to MK at 6:00pm. It was really no trouble at all getting back and forth.