But it says that soldiers were fed 4000 to 5000 calories. Sounds pretty hard to exercise enough to burn that off.
WWI soldiers marched literally for days straight with 85 pounds of equipment on their backs.
Given how soldiers then (and still, largely, now) were young men in their late teens and early 20s... this doesn't sound unreasonable at all. I could sit on the couch and burn that off. Add in setting up camp, digging trenches, hauling heavy equipment through mud and snow? Totally realistic.
According to [1] (which cites [2], page 9), British infantry etc have an average 4,600 calorie intake.