At DL, parking trams, monorail to hotels, etc. runs until everyone is out of the park and back where they need to be. The same is true at WDW. Even with all the growth of the last 32 years, we are still quite remote and it would be totally useless for us to stop our transportation with people still in the parks.
WDW is very spread out, the size of San Francisco.
You will take a monorail or ferryboat to/from MK and busses or your car everywhere else.
The only parks you can walk between are Epcot and MGM Studios, and that's about a 20 minute walk.
Busses generally run up to 2 hours after each park closes (last bus to Animal Kingdom runs at 7pm if AK closes at 5pm, but other busses keep running till 11 for Epcot (which always closes at 9), and they vary for MK and MGM, but always 2 hours after park close.
Parking lot trams run about an hour and a half after the park closes.
HYPOTHETICAL SITUATION:
MK open till 9, MGM Open till 10
If you park at MK, and MK and you finish your day at MGM Studios and you see the second Fantasmic at 10:30, it will be 11pm when you get out of MGM. You will be able to get transportation back to MK (even if the last bus has gone, there are always CMs who can make arrangements.) the parking trams will not be running when you reach TTC. You will either walk to your car or you may get a ride from security. It could be after 1 am when you reach your hotel.
Bottom line is: we can handle just about anything- there's not much that hasn't happened before. Plan ahead, be prepared, but it's silly to play "What if?"
Also: if the park you're starting at closes before the park you're finishing at, it makes more sense to just drive your car between them. Your parking pass is good all day at all 4 parks.
Make sure your worries are in order. I can't tell you how many people I've seen who stress over "what-if's?" about our internal transportation system but don't even know the name of their own off-site hotel or how to get there.
If you're staying off-site, go to Mapquest.com and map out a route from your hotel to WDW.
Have fun.
Congratulations.
Best of luck!
-Steve