You don't have to be staying on property to use the Disney transportation between the Disney theme parks. There are buses outside of all the parks and their destinations are clearly marked on signs. Any cast member would be happy to direct you as well.
The buses come about every 20 minutes although we never had to wait that long for one.
If you're leaving Epcot, AK, or MGM, you walk to the bus area. The travel times between these three parks aren't bad at all (especially between Epcot and MGM).
The Magic Kingdom works a little differently. When leaving the park you have to make your way to the Transportation and Ticket Center via monorail or ferry. Once at the TTC, you can take the monorail to Epcot, or catch a bus to MGM or AK. The Magic Kingdom is the furthest out, so it's around a 15 minute bus ride to the other parks. (someone correct me if I'm wrong

) I would allow about an hour's time to get from Magic Kingdom to the other parks.
On our last day of vacation, we wanted to go back and do a couple of favorites from each park - so we park-hopped to all four parks. We even walked from MGM to Epcot (which I wouldn't necessarily recommend

) It was a very brisk 20 minute walk along the river. Very peaceful, but my legs weren't too happy with me that day!
Hope this helps.