When I was 18-21 and in literally fighting shape I could do 20 miles with a 60-80 Lb loadout today at 54 & 210# and "decent for my age" shape I shudder at the thought of trying to do that... especially without a DI or cranky Gunny motivating me to continue.
I have a 5.11 like yours that I EDC. I used it to do a Goruck Challenge about a year ago, I would have never been able to do it in a full sized ruck.
If that's your go to plan I'd say try doing 10 fast miles in it one day and decide if that is going to be a viable option for you.
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Oderint dum metuant
"Stay with me; do not fear. For he who seeks my life seeks your life, but with me you shall be safe.” 1 Samuel 22:23
“Every man must patiently bide his time. He must wait — not in listless idleness but in constant, steady, cheerful endeavors, always willing and fulfilling and accomplishing his task, that when the occasion comes he may be equal to the occasion.” – Henry Wadsworth Longfellow