you need to practice blogging. it is the most important thing about ultra running. also, you need to plan which bumper sticker you'll get for your car to alert other drivers to the fact that you completed a 100 miler.
There are lots of different ways to train for 100 miles and they depend on what your goal it. Some people can run twice a month and supplement with other types of training and some can run 120 miles a week.
Assuming you are non-elite (not trying for top 10% in a race), here are the essentials.
1) Statistically, the majority people do NOT finish their first 100 mile attempt. You should check past results of the race you are targeting to understand the finish rate and adjust your expectations for finishing accordingly. So running "well" likely means finishing.
2) The two biggest factors are mental preparation and understanding how to fuel (water, salt, and food).
3) In order to handle #2, you generally need to:
a) In the 6 months before complete at least 2x50k runs and 1x50 miler. Races are the easiest venue for this but you don't need to "race." You need to understand your body and practice dealing with discomfort and fueling.
b) You need to train in conditions similar to your 100 miles. This means terrain, elevation change, and temperature. Clicking off 80 miles on the road for months and then going and running a mountainous 100 miles in the heat won't go well.
c) Don't overtrain before your first attempt. There are tons of people who get accepted to Western States and other races and never get to the finish line. Science says you aren't getting any physio benefit past 20 miles, you are only beating down your muscles. So you only do those runs or the "back to back" for the express purpose of training yourself psychologically. Don't listen to any mumbo jumbo about fitness from big back-to-back efforts.
d) Purposefully seek out discomfort. Wake yourself up 3-4 times in a night and then do an early long run. Run starting at 10pm with a headlight. That sort of thing. Run after having a full lunch.
e) Learn how to run with water bottles or vest with bladder. Learn how to eat and fuel methodically.
4) To answer your question specifically, don't run 70 miles a week. Spread out two challenging 2-4 hour runs per week. Runs where the terrain is forcing you into 9-13 min/mile pace and if you have the strength, push yourself for 3-5 miles in the middle of them. Then take a 3rd day and run something more moderate and mix 10-15x100m strides on some soft surface. Spend the rest of your time getting your core strong and maybe some plyo (box step ups or jumps).
Good luck. Running well will mean being prepared mentally, ready to adapt when shit goes fubar (as it tends to do). It's not about running fast, its about not slowing down too much.
I trained with some guys who were into ultras, one finished 2nd at Leadville a few years back. Back to back long runs were a staple. They often started out their training with a long run on Saturday AM followed by Sunday PM decreasing the "rest" to Saturday PM / Sunday AM. Most weeks had a medium long run around Wednesday. The rest of the week was all about recovery. Learning to eat/drink on the run was key, along with being able to shift into a low gear and keep on going.
I trained for a 50 once wrote:
The big emphasis is the long runs. Normally Saturday and Sunday back to back. My 50 mi training got up to 5hr then 4hr. Then through the week you do some (2-3) shorter (marathon long run distance) at Marathon/ Half Marathon pace. Good luck, the hard part is by far mental. It's really draining being out there that long every weekend.
Truly depends on the race and what your definition of "good" is.
Wasatch 100 is not your average 100, but I'll use it as my example.
If you are a life long runner-
Do a couple long runs/hikes 4+ hours and get your fueling down.
The mental component is big but the fueling is MUCH bigger.
I've run Wasatch twice as well as many others: once I did it cold turkey zero training, 29hrs. The other time I did it, I trained like crazy, big miles and had lots of fitness and went in well rested; 32hrs (had stomach issues the whole time)
In the end, I decided fueling was more significant than training. Do both well and have some guts you'll be fine.