It's a little difficult to give you an accurate distance, but typically the disney buses are in the first car park after leaving the park and the DTD are in a second area. It depends on the park. For example at Epcot the Disney buses are also separated into the two car parks so in that case DTD buses are no further at all. However, it is particularly noticable at MK as the Disney buses are able to go to the park itself, whereas DTD go to the Transportation and ticket centre (where you would park).
Overall, they are not a great distance further and are still easily walkable at all parks IMHO
I don't think the Downtown Disney buses go to any of the theme parks any more - only to the resort hotels. Or - are you talking about the bus from the Downtown Disney hotels to the parks?
Ah i took it that you meant the DTD hotel shuttle bus as we are on the hotels board. Apologies if you meant the disney bus service at DTD in which case the above poster is correct- they only go to disney hotels.