You need to decide if you want to train to run a fast marathon or to be fast on alpine climbs. Despite the fact that these two events share some common ground when it comes to having a good aerobic base; one is a relatively high speed event that for fast marathoners lasts for less than 2.5 hours while the other is much slower, longer and hence lower average intensity. Noakes is right that building leg speed and the specific running strength and economy with shorter events will probably make your transition to marathon training easier later. He’s basing this comment largely off his observation that while many middle distance (1500m) runners have made very successful transitions to 5-10km and finally the half and full marathoner no one has every moved down from the marathon to the 1500 or even the 10k and had high level success. Speed and economy are best developed while young.
You need to ask yourself if you want spend the next several years working with a coach on running form and speed. Where are you now with your running? What are your running goals? How will you integrate them with your alpine goals.
Alpinism is a high skill sport requiring a great many years of practice to achieve mastery off. Do this while training to break 15 min in the 5km will present you with many major challenges.
I hope this helps,