[Scott Johnston]

mhsc:  Thanks for writing in with your questions.  Trail/mountain running is an ideal way to build that aerobic base.  I have personally coached some of the fastest mountaineers and alpinists in the world, and all of them used running as their primary modality for aerobic base training.  You might want to read this article https://evokeendurance.com/our-training-philosophy-put-to-a-test-by-elite-alpinists/.

A mix of running and hiking will do the job, and if you are new to running, you will need to be very gradual in your approach to adding volume. You may be aerobically fit right now, but the connective tissues in your legs are not strong enough yet to handle the repetitive pounding that running imposes on them. Run on soft surfaces (treadmills are excellent for this if you don’t have dirt access easily).  It is hard to give you a blanket recommendation without knowing much more about you. But here is a conservative approach: Start with no more than three days of running/week and no more than 30 minutes at a stretch.  Do this for four weeks before considering increasing volume. If all goes well, you can start by adding 10% to each run.  I know this sounds like a crazy, slow process.  The single biggest reason people stop running is lower leg soft tissue injury from overuse.  Most people become impatient and ramp up too fast.

This ‘break’ would be a good time to also focus on building strength for the pack carries.  Box step-ups and downs in a gym with a barbell using the max strength protocol from your 24-week plan will do a good job of building that specific strength.  This will establish a good foundation of strength for when you shift to more event-specific muscular endurance training.

I hope this helps.
Scott