I follow the 24-week mountaineering plan and the 12-week plan is just a shorter version of it.
At a high level, you will have two days of strength training, three days of aerobic workouts, one day of recovery workout and one rest day in a week.
For your aerobic workouts, you can hike up a steep hill (preferred), building stairs, or do these workouts on a Stairmaster/Treadmill in gym. You don’t have to run.
Hope this is helpful.