Great question. The 25% is only suggested as a starting point. If you dig into our ME writings on the site you will see multiple references to the notion that the pack weight needs to be adjusted to each individuals strength. Stronger people will use a much higher % of BW. You’ll load up your pack to the point that when hiking steeply uphill (skinning is not steep enough for these workouts to be there most effective. The skins slip) local muscular fatigue in your legs is the limitation to you your speed. Not your breathing or heart rate. When loaded appropriately you will be able to carry on a conversation but your legs will be at their limit with a light burn going on. It is sustaining this low level burn for a long duration that you are looking for. Take a look at this article if you have not read it already. It should clarify this.
If you have more questions please let me know.