The journey is not a bad one to take. Me and the DW do it every visit early on in our stay.
You looking at around three to four hours depending on your run.
We always use the florida turnpike for the entire journey, the turnpike is a toll road and comes to around $14 total one way. Depending on what car you have, we usually have a convertible and use around $40 worth of fuel.
there are 3 or 4 service stations along the turnpike that have starbucks, toilets, burger king etc.
I like the turnpike as i can set the cruise control and take the trainers off and relax all the way to miami!!!!!!
As a side note, avoid sundays if miami dolphins are playing at home, they close the turnpike off at then end of the game allowing the fans to filter straight out on to the turnpike but means everyone else has to do the I-95 route which can be congested.
It is an easy journey on the turnpike (if a bit boring). The exits for Kissimmee/Orlando are well signposted, although saying that I cannot remember what the exit numbers are.