I'm in my fifties, not sure if that qualifies as "older." My knees let me know when I'm five pounds overweight, let alone fifty. I quit running because of the pounding, now I roller skate for cardio fitness.

Overloading for a backpacking trip once or twice per year isn't likely to build muscle long term. It's too infrequent. Far more likely that it will cause joint damage if you ask me. Besides, I'm out there to enjoy the trip, not to be a slave to an overweight pack.

I don't see someone who hikes 10 to 15 miles per day with 20 to 25 pounds on their back as being an out of condition wimp, but that's just me.