@Edgar Glad that you are sorting out the heel issue!
You raise a great question about the rest step. We could go down a rabbit hole with the discussion…
On Denali, you’ll need to be strong with a rest step and with a more normal gait. Using a good rest step can sometimes make sled pulling feel more difficult. But the upper mountain demands a good rest step. So you should train for both situations:
Early in the program, the training is general and the terrain doesn’t really require a rest step on most days. If you are finding terrain steep enough, and you need to pace yourself enough to stay in your target zone, then you can employ a rest step or partial rest step.
Later in the program, as training is more specific, it could be beneficial to do rest-stepping when you are in the terrain that demands it, and wearing footwear that is more specific (full shank mountain boot). Heavy weighted carries will slow athletes down, and those session might bring some of the MTG athletes down to a rest step pace if they are on steep enough terrain. This will be different for everyone.
On the mountain:
Knowing how to move a heavy sled using a “modified” or “minimal” rest step is important on Denali.
A normal rest step will usually let your sled come to a complete stop, and it can sometimes take a fair amount of effort to break that friction and get it moving again. Having a slow gliding gait or a modified rest step will keep the sled from stopping completely and will sometimes feel more efficient than a proper rest step. Again, this will probably affect each athlete differently, and conditions and sled setups will be different too. Those with a strong preference to rest-stepping sometimes prefer to allocate more weight to their shoulders because a “sticking” sled can be frustrating and difficult. This is not an option for most climbers.
Above 11k you’ll usually feel it’s less of an issue, because of the tactic that Eicke highlighted. Some teams reduce the number of sleds per rope team above 11k, so it’s possible that you might already be without a sled at that point.
Above 14,200′ the rest step will be very important. It would serve you to train with it whenever steep terrain, heavier training loads, and/or longer sessions call for it. If some sessions don’t call for rest-steps, then don’t force the rest-steps – these sessions will end up more relevant for the sled-pulling situations.
Without the challenge of altitude and really heavy loads, most athletes will need pretty steep terrain for a rest step to come into play in training.