Experience From - Shawn McDonald ,
Well, I would suggest lots of long training runs at a conversational pace, perhaps building up to 40 miles by a few weeks before the 100k race you run. The 50k in Sept. would be a good test of how you are doing in the training and should not be too much more difficult than a marathon. If you have been running lots of weeks lately of about 40 miles, then you have a good base to build on. Gradually build up weekly mileage over the next year into the 60's and 70's with an occasional "rest" week of 30 or 40 miles with no long run. That will be enough mileage to get you comfortably thru your first 100k race. It would also help to phase in some faster running, once per week, like a tempo run or fartlek or hill repeat session. Maybe do this mid-week if you run long on the weekend.
You can get a rough prediction of your 100k time from your marathon time. I like a factor of 2.8 times the marathon time equals your 100k time. The distance factor says 2.4 but you run slower in the 100k. It would be very likely you would run in the mid-8 hour range if you can get in solid training over the next year, as in #1. If you get as fit as you were for your pr marathon (along with some experience running ultras) the time might be more in the mid to high 7 hour range. A good base goal for your first 100k might be to finish at better than 9 minute pace (9:18) and a good challenging goal might be sub 8:30. If you run a 50k race next summer about 2 months before the 100k that will give you a better idea of what you might be able to do. In that case, use a scale factor of 2.2-2.3 times your 50k time to get your predicted 100k time.
An example program for the Comrades Ultramarathon (54 miles) is on the web at: http://www.halhigdon.com/ultramarathon/ . The program here is mostly ok, although they have you doing a pace run on Sat. and a long run on Sun. which I'm not sure I like. Also, you might like to do a few/couple long runs of more than 4 hours in prepping for the 100k. The mileage you have been doing is similar to where their program starts as a base. I would recommend a couple of races in the months before the 100k, maybe a 50k and an 80k/50 mile with the last 6-7 weeks before the 100k race free, and then use a 2-3 week taper to rest for the 100k. A few shorter races (10k-half marathon) can be added in for a change and to see where your speed fitness is. A marathon can be used as a quick long run to practice running at 100k race pace and holding back early. Note that if you go for the sub 8:30 goal then early on in the 100k you would be running at or a shade faster than 8:00/mi which is quite a lot slower than your marathon race pace.
There are a few people on the list who coach ultrarunners. You might also try to contact your national ultra association or track assocation; they would know of any coaches in your area that specialize in ultrarunning. A couple of good books on Ultrarunning are "Lore of Running" by Tim Noakes and "Ultrarunning: The Next Challenge" by Tom Osler and Ed Dodd. I think both list sample training schedules and discuss many aspects of ultrarunning including nutrition and mental approach.