My "regime" for the last ten years or so is to hike 5-ish day with weekly totals in the 30 mile range. These are single track trails with a fair bit of climbing (and descending). Hikes during the week are 3-5 miles, the weekend usually brings one longer hike of 10+ miles (usually in the mountains with a backpack). Interspersed are longer backpacking trips summer through winter.
I strength train twice a week- Wendler 5/3/1 revamped for me doing squats/bench on one day, deadlifts/overhead press the other. I work the core during those workouts as well as accessory lifts of pull/chin-ups, dips and lunges.
The key imho is building the aerobic base, this takes months (maybe years?) and then keeping it up. Even elite athletes are taking advantage of the newer studies showing that the vast majority of your mileage should be in the moderate heart range where you know you're exerting, but can still carry on a conversation. Some go much deeper into the zones of training- check into uphill athlete.
Consistency. I see too many 6 week this and 12 week that; training should be year round. Make it a routine.
I just finished probably the toughest trip of my life -snowshoeing a 100 miles across the Bob Marshall Wilderness. It was arduous (and a bit too dangerous in honesty) and I'm beat up for sure. But satisfying knowing that only a small handful of folks would be capable of a trip like this and far, far fewer at age 64
