I think it entirely depends upon when in WWI you are speaking towards. For instance in the beginning of the war from August until September the French and British on the left flank of their lines were in almost constant retreat almost from the onset of the war. Some credit can be given to Joffre's leadership as it prevent a route. In these early months progress was measured in miles. This retreat was so extreme that there was some reported insubordination by Lanrezac, who continued to refuse an order to attack Bulow until Joffre threatened to have him shot and this was just in the first few months when moral would still be assumed to be high! At this time we could make the argument that all three armies showed massive endurance because of how fast they were retreating/advancing, but I do not think that is what you meant by the question.

Later in the war, in April of 1917 we have a utterly massive French Army mutiny which effected nearly half of the divisions on the western front. It took major concessions from the new general in command, Petain, to ensure that the French could even continue the war. One of these concessions was a plan to incorporate unit rotations for front line soldiers away from the front, one of the first instances of such a thing. The extent of this mutiny and particular details are held confidential until later this year for historians!

I know this doesn't fully answer your question and I hope that there will be a second poster with a competing view, but I wanted to offer an alternative view to shed light on the extent of WWI soldiers endurance and if it can be categorized as something spectacular or merely a product of our current historical narrative of WWI. As always, I will update this for formatting or spelling errors.

Sources : Capital Cities at War: Paris, London, Berlin by Jay Winter

The First Wold War by Hew Strachen

The Western Front 1914-1918 by John Terraine