I take it you are staying at FW.
The Green launch (WL/FW/MK) boat ride is not long. I'd say it averages around 10-15 minutes.
There is also a Blue launch which runs WL/CR/FW from 6:45 am - 10 pm every day. There are generally 2 boats running, 1 clockwise and 1 counterclockwise. If the Green launch has not yet begun, you can take this boat and walk or monorail to MK. If you have an early PS, this is a good way to get to MK early.
There is now a direct bus from WL to MK. I'm not sure if it starts at FW. I would think so since your internal (orange) bus makes at WL stop. I'd ask someone in "purple pants" when you arrive.