The walk is easy (for us anyway) and very pretty. You enter CityWalk behind Margaritaville. This is closest to IOA. It isnt as close as HRH is to USO but its not much longer either. 10-15 min depending on your stride.
RPR is not across the street from the parks. I walked to City Walk is six minutes from Tower One of the RPR. You can also take the water taxi to City Walk. We always found it easier to just walk. We did take the water taxi back some nights when we were SOOOOO tired.
The walk is not too bad, but it always seemed farther and longer at the end of a long day! We normally caught the boat if it was there, but walked if we just missed it.