These kinds of relays are more intense than doing the thing by yourself. If the race is 24 hours you might each race 8 times. That’s 8 times 90min of near max effort. I think you’ll need to include a good bit of uphill intensity and down hill fast running as well. I’d build a base for both the up and down with our ME progression as explained near the end of this article. Everyone who uses this say’s their legs are bullet proof.
The main thing to keep in mind is that the relay with it’s rests will completely alter the character of this race from an Ultra event to 8 x 90min hard efforts. For this you need a big base but to race fast you need intensity. It needs more complex training than a normal ultra race.