It was easier to be in shape back in the day because "junk food" wasn't a thing. Companies spend millions and billions of dollars using scientists to come up with artificially flavored food that appeals to our taste buds and brain receptors.
Old school people ate chicken, beans and greens. Maybe some peach cobbler on Sundays. Food had less hormones in it.
Having said that, I'd take a modern soldier over a WW2 soldier even with the same equipment. Saw a model of the "average" soldier back then. Those dudes were small. Probably more fit but even a couch potato today could man handle one of those dudes back then. Problem is that the old school dude could do a 10 mile hike in full gear with ease. Kids today...