In Ireland:
Tough walks (would think about taking a cart) - European Club, Ballybunion Cashen
Tough but enjoyable walks (would never take a cart) - RCD, Ballybunion Old
Reasonable walks - Lahinch, Portmarnock, Doonbeg
The biggest factor in a tough walk for me is the elevation changes, especially between holes. The wind can play a factor as well. For example, at Ballybunion I was exhausted by the time we reached the sixth green just from trudging into a head wind for the first third of the course.
TEC had one additional factor that pushed it to the top of the tough walk scale, the amount of time spent crawling up and down the rough areas looking for errant drives.