I’d suggest planning it out in a spreadsheet. Start with you’re last BIG race prep weekend ~3 weeks before your 100 miler and then work backwards gradually bringing the workouts inline with your current fitness/ability. Be methodical in how you progress the workouts/volume from week to week. Try to plan the overall mileage (though, I’d recommend tracking time), intensity and volume of your workouts, and the vertical. Factor in recovery weeks ~1 per month.
How I would do it (disclaimer, I’ve only ever done marathons, but I’ve recently been watching all of the ultra YouTube personalities and daydreaming about these races). I’d do a 3 week cycle for my weekend workouts: 1. Easy long runs on Sat & Sun. Immediately after the Sunday run, I’d do a body weight exercise routine (e.g. single leg squats/calf raises/deadlifts etc. ) 2. Easy long Sat. & marathon-type workout Sun. (Eg 2 1/2 hr run with ~30-45min @ marathon pace near the end). 3. Trail “outing” weekend, getting geared up both Sat & Sun and spending the better part of the day walking/hiking/jogging but not worried too much about distance or pace. For each weekend, Saturday would be easier than the Sunday effort.
For the rest of the week, I’d run at least 6 days per week, do a hill sprints session (8 second reps) each week, alternating weeks I’d do either a flat/rolling hills tempo or a treadmill uphill tempo workout. If I had the energy, I’d also do a VO2 max workout like your 3-minute hill reps (though, I’d likely alternate 10k pace weeks and 5k pace weeks while varying the length of the reps). The rest of the running days would be easy/recovery days. I’d make sure to do body weight exercises 2–3 days a week on the “hard” days, with an emphasis on single leg calf raises & single leg squats.
So an outline of the week: 40-50min easy, 40-55min with 8-10 second hill sprints (build up to 10 or more), 70-80min run with a 20-45 min tempo (rolling hills or uphill treadmill), easy, 5k or 10k effort hill rep workout adjusting, then one of the weekend sessions
Personally, I’d want to at least do the following in training before feeling ready: 1-2 100 mile weeks, 1-2 weeks achieving a vert >= to the race, a trail “outing” weekend spending 7-8 hours each day, and a long run weekend were I Run 45-50 miles.
BUT, since you don’t have much of a running background, I’d suggest you don’t worry about mileage. Focus on staying healthy, running consistently and having some variety in you’re training. Unless you’re an elite, it seems like these races are mostly just glorified hikes with some jogging. Run to have some fitness, but the trail “outing” days will be what get you through. Don’t worry too much about your running fitness. Even when I’m fast/in running shape, walking/hiking is a very different exercise & I’ve gotten pretty beat up by moderate hikes. Spend some time hiking vert (walking on a treadmill set to max incline for an hour+ ) so your calves get used to it. The long trail outing days (and the high reps of single leg squats) are necessary to get used to all of the eccentric work. Practice wearing your gear, and eating calories/hydrating while doing long bouts of easy/moderate exercise. Get to the starting line healthy & having figured out how to take care of yourself (e.g. no blisters, nausea etc) and you’ll be in a good position for success.
Also, since you’re new to running the 7+ mile “easy” days are likely interfering with your recovery. Keep your recovery runs under 45 minutes. And having an easy day after a rest/off day doesn’t make sense to me. Good luck!