I'm 54. My ex husband is the same age and was career military.
At some point in his career, they changed the PT test standards for younger military members because young people growing up in front of TVs and computers didn't have the stamina his generation had at the same age. Meanwhile, they did not lower the test standards for him/his age group. He quite resented it.
In other words, this is Federal policy acknowledging this change.
Last I heard, the military is having increasing difficulty getting qualified applicants because young people today are simply out of shape compared to previous generations. I've also seen articles that say we increasingly import construction workers because middle and upper class young white males no longer take construction jobs during the summer in high school and college like they routinely did when I was that age.
A lot of it is stamina. Both physical and mental. Rural peasants are really tough in a way that is extremely foreign to suburbanites.
My godfather (indigenous peasant in rural Mexico) spent some time in his youth carrying two 50-pound bottles of liquor on his back all day long up and down mountains over a rough trail. I get winded if I try to carry one of those bottles for a block of flat sidewalk. If someone asked me to strap two to a tumpline and hike up a mountain I would call them crazy.
He spent other years working on coffee plantations under thieving brutal racist owners. And continued working on his own small plots of land every day up to maybe age 70, even as his body was thoroughly worn out.
Now his grandchildren are white-collar professionals.
This kind of hard manual labor is as demanding as any high-level sport. It takes years of (mental and physical) training to be able to handle it without falling apart.
Do it as much as you can for month and see how far you get.
I'm 39 and for the last 6 months I've been doing a job that involves lots of manual labor, it's gotten easier, not harder. I guess I was in relatively decent condition at the start though, if you consider the US average.
Fun that your edit added "willing" while I was writing this.