Advice needed on back to back runs vs one long run

I have a 100km ultra booked in for October and have seen some conflicting advice on wether a single long run or back to back runs are better. I’m looking for some advice on which approach is best to take.

If you have experience I’d love to know which made you feel better on race day and throughout training.

For context the only other ultra I’ve done was 50km, but plenty of half marathons and marathons, and i’ll be starting the training block for this ultra off the back of a marathon block.