In reply to Rampikino:
You’ll need to eat.
I’ve done Endure 24 with a team of 5, they’re 8km laps.
15k sounds a good balance between running and eating/recovering.
Make sure you either prep your food before your laps or have someone ready with food so you can eat as soon as you finish your laps.
Take it easy from the outset. If you do 40miles each you’re going well.