I work at a commercial gym. A few months back a 59 yo woman came to me asking for strengthening her legs and hips for a 46 miles ruck. She started doing the SSLP and progressed until she needed 3s for bench and press, and a light day for squats. I just got to talk to her, and despite having blisters all over her feet, she rucked the entire course without feeling tired.

Also, worth noting, a couple of years ago I joined a 3 mile mud race having not run a single time for more than a year. The race was Saturday and Friday was TM intensity day. Squatted as usual and finished the race with no difficulties.