My longest walk was in 1972 - It was the Miles for Millions walk in Hamilton Ontario, 35 miles or 56km. The money donated by pledge was for Hunger - I'm not sure what organization was used (I'm thinking UNICEF). My total pledges if I completed the walk was $50.
The reward? To cross the finish line even though it wasn't a race. I was too young at 16 to appreciate the importance or purpose of the walk. It was all about me and my feet. I wish I could say I had starving people on my mind as I struggled to get through it.