yes we will have a car. how easy is it to drive over?
That depends.
Let's say you are at MGM - its pretty close to POR. You got there early, so you are parked pretty close. And when you decide to leave, you are at the front of the park. It will probably take you maybe 30 minutes before you are eating at POR.
Lets say you are at the back end of Epcot - maybe somewhere around the Italy pavilion. Its going to take about 20 minutes to walk to the front of the park. You got there a little late and its a busy Epcot day, so you may need to take a tram to your car. But Epcot is still pretty close to POR, so maybe you'll be eating in 30 minutes.
Let's say you are at the back end of MK. Its a small park, you can get to the front in less than 15 minutes - unless the parade is going on! But from the front of the park, you may still have a 20 minute trip to your car by monorail or boat and tram. And POR is about five miles away from there.
Then there is the potential of a wrong turn. We aren't good Disney navigators (and therefore don't tend to drive on property) since we often find ourselves lost.
Add in the equivalent amount of time to get back - if you are doing this at the end of your day, you'll only have half the travel time (but then you'd probably be better off off property) - but if you are planning on reentering the parks, you'll have to get back.
Even with a car, I'm guessing that from the time you say "lets eat" to the time you are back in the park on a ride will not be less than an hour - and closer to (or in excess of) two. Which is a lot of time at Disney for a counter service meal - particularly for a first trip. (Veterans of Disney often do odd things, like waiting an hour to eat at Beaches and Cream after getting there from the MK (a half hour trip at best) - but they understand the trade off - and they've already ridden Pirates and if they miss it this year, they'll see it next if they want.