Sounds like an oxymoron - and it is - but here is a question for serious ultra guys.
What are your favorite workouts to sharpen up for a 100k distance after the peak mileage/race specific block is complete? Time frame 3-6 weeks out. Do you pepper in VO2MAX, hills, 200/400/800/1000/Mile/2Mile/31-Mile repeats? Thanks.
I'm not sure sharpening is the right word but ultra success can usually be boiled down to mileage and vert. As far as workouts go in the last 3-6 weeks, it's all about the long runs and a steady dose of medium long run. Mixing in threshold work, marathon paced work, and some uptempo running is fine. Some VO2 max stuff will help but if you're racing on hilly trails you might just want uptempo 10:00 reps in a 4 hour long run.
I don't think ultra training needs to be too complex.
Maybe in a 100 mile week you do a weekly workout of 6 x 10:00 uptempo working down to marathon pace with 1:00-2:00 jogs. Hills another day during the week. On the weekend do a good MLR, maybe 12-15 miles, maybe the last 3 working down to MP. Then a long run of 20+, playing around with some faster running.
I assume this is for 100k on roads.
Top runners do marathon training with a long run of about 50k on the weekends.
Less focus on speed, more focus on the long run.
But at the end it comes down to perfect pacing on race day.