The workload difference is in the fact that with walking lunges, each langing leg works a little harder in controlling the landing with each forward and backward "walk", as well as propelling up and one of the feet losing contact with the ground, even though at least one foot stays in contact with the floor at all times. Obviously, this is different from standard lunges where both feet remain in contact with the floor at all times. Also, there are subtle balance issues that change when you are performing walking lunges rather than grounded lunges; your core needs to adjust and control itself with each "step".
A good variation to the walking lunge for the space-challenged can be found in the Legs And Glutes workout; remember how you lunge forward, then back, then go into those murderous "explosive lunges"? Just do that forward-and-back lunge, keeping one foot grounded and the other foot doing the forward-and-back walk; complete X reps on that side, then switch and do an equal number of reps with the previously walking leg grounded and the previously grounded leg walking.
A-Jock
