If you walk 3 miles per day, then a quick walk to Settlement or Outpost will be easy for you!!
Also, when you bring your bikes you have to worry about which end of the campground you parked at when returning from the parks. Also, if it is pouring down rain when you return from the parks, you have to ride your bikes in the rain. If you don't have them, you can just catch a bus to your loop.
For example if you ride your bikes to Outpost in the morning to catch a bus, then during the day hop over to Magic Kingdom and take the boat back to Ft. Wilderness, you'll be brought back to the Marina (Settlement), then have to ride a bus or walk all the way to Outpost to retrieve your bikes.
From Loop 1500 it is an easy walk to either destination.
Hope this helps,
Carol