The biggest caveat here is that ultra training can vary significantly depending on terrain. Prep for a mountainous 50-miler/100k (i.e. Canyons) can look drastically different than a 'racetrack' 100-miler (i.e. Hennepin.) Vert and trail specificity needs to be layered in appropriately. When vert and climbing is a factor, adding this in and going by RPE and HR is a good choice.
I'd recommend switching up long runs between: (a) a 'shorter, regular' long run in order to allow the body to recover, (b) back-to-back (i.e. Sat & Sun) long runs as people have mentioned, and most importantly (c) an over-distance long run.
Somewhere along the way in ultra training people got this idea that 'back-to-back' long runs were the key when in actuality, from a mental and metabolic standpoint, doing a single 'longer' long distance long run is better preparation for day-of. The caveat here is that you have to space these out appropriately so that your body can respond to the stress this places on it.
Aside from just building volume and getting a handful of single-long and and back-to-back long runs in, any experienced ultra runner would tell you that there is still benefit to putting in work at threshold and sub-threshold (Vo2-ish.) The reason is the same as if you were marathon training. This work well help to improve running economy - the stronger and more efficient your stride is, the further you'll be able to run until exhaustion. Most all pro ultra runners still engage in these sessions (though not with as much volume since you also need to prioritize recovery from your overall volume.)
I'd venture to guess that for experienced runners and those who have put in significant milage in the past, there's a better chance that an inadequate aerobic base will not be the primary limiting factor in you their first one hundred (although more is usually better), it's like to be under fueling. Carbohydrate/electrolyte intake is so different than a marathon - you need so much more and if you're not used to training to get this in while running, you'll surely bonk early. Performing up to your potential over 100mi is maybe two-thirds mental/aerobic and one-third eating contest. You might be used to taking in 50g of carbs an hour during the marathon, but are you trained to get down 80-100g+ in order to chip away at the deficit you'll be building? Moreover, everyone sweats at a different rate - if you'll be running 100 in warmer temps and are a salty sweater, what is your plan for consuming high amounts of sodium.
TL;DR: don't neglect single 'longer' long runs, don't neglect threshold/sub-threshold work, and begin to practice taking larger amount of carbs/sodium midrun than what you're used to as that will be crucial to sustain energy.