Ah I’ve been audaxing 12 years, with 10/11 of those years including the longer ones.
Do not think that speed does not matter, it does, but it’s your all day pace that matters. You still want to include the high intensity and not just think you only need to do the Z2 for audax. Z2 should still be your foundation but do include the intensity as Polarised does. When you hit a long or steep hill you’ll be glad you’ve done the intensity.
Because they are such long durations even small increases in speed make a difference.
At 18 km/h moving average you have a maximum of 2 hours 22 mins stopped time, and 11 hours 8 mins moving.
At 20 km/h moving average, you have maximum 3 hours 30 mins stopped time, with 10 hours moving.
At 22 km/h moving average, you have maximum 4 hours 24 mins stopped, and 9 hours 6 mins moving. That’s 2 hours less riding, than at 18 km/h, plus much more time to stop eat and have a break and digest food.
These are true moving averages rather than the ones reported by GPS set to auto pause when stopped at junctions etc. Auto pause often inflates moving averages by 1 or 2 km/h, good for the ego, but not your calculations for time limits.
The other mistake made in audax is not eating and drinking enough. This is often because what you can get away with on a 3-4 hour ride does not scale up at audax durations. Thus performance can drop off a cliff later on. You often see this with sportive riders new to audax. They speed off into the distance early on, riding tempo and above, only for you to pass them as they crawl along 6-7 hours later, having burnt their matches and realised gels don’t work for the duration.