You should run at the highest sustainable pace you can. 🙂
This isn’t answerable. Or it is if you accept that what someone recommends may not work for you. Any answers will depend on fitness, experience, pain tolerance, and most importantly, duration.
“Your last ultra race” isn’t specific enough. What distance? And more importantly, how long did it take the person that responds to your question?
More specific to you:
* What is the distance and gain of the race that you’re training for?
* When you do a long training session, what is the distance and gain?
* How long does a long session take and at what heart rate (relative to AeT)?
* How many of those long sessions can you do day after day? Can you do enough of them to add up to the distance and gain of the goal event?