Mengu wins a cookie.
In the latest adventure posted on Dungeon Magazine, Rescue at Riverroar, there are two skill challenges regarding exhaustive movement for too long. Not counting the "fluff" for the different terrain involved in each of those skill challenges, the endurance check portion is the almost identical -- basically DC 10 Endurance check. If no one of the group makes the check, everyone looses a healing surge.
p18 and p20 per the magazine's page numbers (or p15 and 17 per the number in the pdf file)
The other two checks involved in the skill challenge are Nature to survive the land and avoid natural hazards (rock slides, weak overpasses, etc) and Perception to stay on the trail they're following or to possibly notice a shortcut -- failure on either of those adds to the time it takes from getting lost, etc)
So, pulling from those two examples, it seems like for your situation where they are just doing extended forced movement (if they don't have the possibility of getting lost etc) then just an endurance check for every hour. They can't assist (or each person can only assit on other person). And for each hour's check, if one or fewer people succeed then everyone looses a healing surge.