Why do you train for a marathon with interval training? Only the Slow Twitch musculature is needed for the marathon and you don't address it with interval training.
Would be cool if someone could explain that to me regarding muscle fibers.
Interval training has little to do with the muscles in marathon training, I think. The intervals are relatively long. At that time I ran 400m-1000m, it's purely about speed training: pulse up and regenerate ;-)
Why do you train for a marathon with interval training?
To increase the running speed. That is why you do tempo training once a week during marathon training (e.g. Increases, sprints or interval training). In the marathon it is not just a matter of reaching the goal. The winner was the one who reached the finish first - and that's the one who ran the> 42 km the fastest.