You can't really judge Cooper because of his age and his smoking. plenty of people, including myself are in good shape who have smoked for years and able to carry on like everyone else. then the fact of feeling like an escaped convict with dogs on your ass will get you in gear pretty quick. no hunger is going to bother you unless you stop for a long period or feel safe enough with the surroundings that would bring back the hunger and the cold.
Put a guy out there a hundred pounds over weight and never seen a treadmill or his feet in ten years will run out of gas in 20 minutes without gear..
Of course you can make some reasoned assumptions based upon the real-world challenges he faced.
I'm 55 y/o, 6'3", weigh 205 lbs, don't smoke, and am in pretty good shape. That said, put me in the middle of no where at night when it is cold and raining wearing loafers and a suit with a raincoat, load me down with at least a 20 lb bag of cash, and force me to start walking through brush, and mud puddles and sticker bushes all while trying to avoid being seen before the sun rises and I'm not getting any further than a mile or two before I decide, "I've got to stash this shit somewhere or else I'm going to get caught or die out here."
Additionally, I'm going to be thinking that I can get back to a warm restaurant or somewhere else a lot faster if I'm not trying to replicate some survivalist challenge.
It's smart, but more importantly, it's probably going to be physically required. I'm telling you, just the act of searching for about 6 hours or so each day during filming wiped us all out.