When we were there in '77 you took a tram to the marina to get to River Country and the launch to the MK. There was no other place to go.
In '99 the trams were gone, but this was when there was more than one park and the buses for the parks looped around the park. There was also the two buses that just looped the park running in opisite directions and one bus that took you to the TTC so you could transfer to a bus to one of the water parks and maybe DTD.
I'm assuming the buses starting running when Epcot opened.