Every rider no matter age or fitness has a self imposed limit or willingness to do more. tho the sled may be easier to ride does not mean that its the only factor in your scenario
Larry, although I DO agree with the original post from savagecanuk, I think there is a LOT to what you are saying.
To relate with a totally non-sledding example...
I waterski a lot in the summer. I dig a good short-line slalom pull at 32' off and 36mph...I hit it as hard as I can. I like my hip to touch the water when I carve. The river at Seeley Lake is about a 1.5 miles long...I usually make 2 or 3 laps before I ski into the dock. That's about 3 to 4 miles of skiing straight. About mile two...it's starts to get tough.
I can be totally whipped both in my legs, arms, back AND cardio.
I'm bending at the waist in my turns and wake crossings...headed for a big ole fatique-induced wipe-out if I'm not careful.
But I ALWAYS seem to be able to pull out for one more set of turns (usually about 8 or 12 turns in my set).
When I'm the most tired and whipped, I can usually do my best skiing. WHY? It's not physical strength or stamina that gets me through.
It gotta be mental strength more than anything. I'm usually happiest when I'm the most tired, yet still cranking the best turns and getting the slickest wake crossings.
One other factor...MUSCLE MEMORY serves BOTH of these issues between cardio and mental toughness.
My Dad is 64 and has been riding since 1976. I'm 39, been riding since the same day he started.
I'm in FAR better health, condition and shape than he is.
But he and I can go out on a ride, get the same mileage, do the same things...and neither of us is more tired than the other. We ride the same model of sled (one having WAY more mods than the other, LOL)
Muscle memory.
So Savage...which one of you has more lifetime sledding miles?