I'd skip Animal Kingdom all together, it's seriously boring, hot, and not much to do. If you get park hopper passes then I guess it'd be okay to spend some time one morning there, but nothing more than that.
Universal (especially if you get tickets for both parks) can easily fill up a full day.
Otherwise I agree with ProfJohn. Take your time at the other three parks as there is a lot to see and little things beyond just standing in lines for rides.
As far as walking strategy, I'm not sure about Magic Kingdom but the last time I was at Hollywood Studios they actually had signs up with the current wait times on the major rides. So if Magic Kingdom has the same, you could just pick the shortest wait big ride you want to do and then go do that whole area first. Then work your way around, or move to the next area with the shortest wait time.
One of those days you should also check out the Disney Downtown Marketplace (if that's what its called these days). It's basically a shopping/dining area built around a lake. It's free to enter and is a nice relaxing area to walk around and have some dinner. You can take the buses from any of the hotels or parks to get there and back.
And since I'm older now and everything revolves around food, here's a couple food tips:
Magic Kingdom: Since you are going with your girlfriend, first thing early in the morning setup a reservation for Cinderella's Royal Table which is the restaurant inside the castle. (
http://disneyworld.disney.go.com/dining/cinderellas-royal-table/). Yes it's expensive, but the food is good and the setting is great for couples.
Epcot: Lunch at the United Kingdom pavilion, there is a counter service spot with some damn good fish and chips and its cheap (for Disney). Sit off behind the building by the water.
MGM/Hollywood Studios: We really enjoy the SciFi Dine-in Theater. Food is standard park food but you sit in booths made to look like old cars at a drive-in theater with random old scifi'ish stuff playing. Well worth it.