You only have to pay parking for the day if you are staying off site, and only once for the day. If you for example want to go to other park in the afternoon, just show your parking ticket you paid in the morning in the other park.
You can park outside of any park.
If you are staying on property, the parking is free. Besides, you will have free transportation Buses, Ferry, Monorail, depending in which hotel you are staying, from your hotel to any park, from park to park, or to Downtown Disney, Water parks, etc. If you are staying off site, you can use the transportation system if you have a hopper ticket.
In any parking lot you will have free trams to get you closer to the park entrance, so you don't have to walk too much. (Remember to write down where you left your car, the parking lots are very large)
When you go to the MK, you must park in the TTC, then you can go to the main entrance by ferry or monorail.
Hope this helps.