Yes, there is an internal bus. But we've never used it. The longest walk to OPR is from Trinidad South, and it took us only 7 minutes to walk it. No big deal. And those are the pirate rooms, so unless you booked those, you will be closer to OPR.
We have never found the walks to be very long at all. Both times we have stayed in a standard room location it has been a less than 5 minute walk to get to anything.
Some of the buses start in Martinique, go clockwise around the resort, with Barbados being the last stop before the bus leaves the resort.
Some buses start in Jamaica, go clockwise around the resort, with Trinidad South being the last stop before the bus leaves the resort.
So whichever stop you are closest to, you are never always going to be the first or the last stop. If you do happen to get on at the very first stop, it only takes about 8 minutes to make the rest of the stops (some are very close together - like 30 seconds apart).
Once the bus leaves the resort it is 3 minutes to DHS, 5 to Epcot, 9 to DTD, and 11 to MK and AK.
CBR is in the "Epcot area".