Loop 500 is close to the bus stop and is a close walk to the marina.
Please be sure to allow lots of time for riding the buses. You will need to ride the bus to the front of the campground to reach buses for Epcot, HS and AK. This may mean a 20 minute wait for first bus then a 20 minute wait for park bus then as long as 20 minutes for the ride to the park.
The biggest mistake we saw new campers make was that they rode the bus to the front bus stop to get to Magic Kingdom. You MUST ride the boats to MK unless the boats are closed due to fog. There is no sign at the front bus stop to tell you this. The boats start running 30 minutes before MK opens so if you have early ressies at MK, you must ride the boat to Contemporary and then walk to MK.
If you are in Loop 400 then the bus stop is over near the stables and Pioneer Hall. Be sure to check out all the bus routes so you get the fastest bus.
To get to some places, we found it easier to ride the boat to MK and then hop a bus to another resort or park. Example to catch dinner at Yachtsman or World Showcase at Epcot, we would ride the boat to MK, and take a bus to the Yachtsman where we could walk to the International Gateway to World Showcase. Also, you might take the boat to Contemporary and catch the monorail to TTC then monorail to Epcot.
Bottom line: Always allow at least an hour to get around the World on the bus/boat system. Watch out for the crowds leaving on the boats when Hoop de Doo lets out and for the pre-parade/fireworks crowds for the boats.