The best course I've played that would be a tough walk day in-day out is TPC Wakefield Plantation. Its got some real big hills on the course and some long walks from green to tee in a good number of spots. I'd say Wakefield is a 4.5-5.
An ardent walker would have to be nuts to join that place.
Now the toughest walk I've ever had was at Tobacco Road. That place has a fair number of hills and a couple of long walks. However, the day I played it, the temp was hovering around 100, with no wind and near breathtaking humidity. Heat index was probably 115 or better. The Pit tends to have mad high temps as well. I'd give TR a 5.5-6, and say that if played in the summer months, its probably the toughest walk I've ever had.
I've never understood the complaints about walking TR (granted I've always done it in the winter - I'll give you that), but the walk itself (provided you're carrying your bag - using a trolley is not recommended) isn't that bad. Um, to me. I know I have an extremely high threshold.
Same goes more for Bethpage (Black), and moreso. No big deal on that walk.
Royal New Kent comes to mind. Certainly not walkable
Done it. Stonehouse, too. Each round was the 2nd round of the day for me.
I'm surprised that nobody has mentioned Highlands Links in Nova Scotia. Some of those green-to-the next tee box walks are legendary.
It was mentioned above.
My vote goes to Kapalua - Planation, as well. I did the "Mercedes walk", riding from 5-6, 8 tee to green & 9 green to 10 tee....although for the life of me, I don't know why they cart it from 8 tee to green. That walk isn't as bad as going from #4 tee to fairway LZ or the same on #10.