In reply to iusedtoclimb:
You said you trained for a 100 miler and 70 miler but did you finish them, and in what state? The thing you will hear from all coaches is that training is an n of 1 - basically what works for you is likely to be very different in many subtle ways to what works for others. Going on your past experience is a good way to decide how to adapt your training.
However i find that not dropping out of the game helps - so how much is your 'next to no running'? Keeping a consistent base would be really beneficial so try not to let your number of runs drop too much even if the volume and intensity die off. CV fitness is relatively easy to get back but the muscle memory and bio-mechanical adaptations that take place over years of consistent training are much harder.
One change i've made to training this year is basing it on effort rather than distance as especially on the trails distance and pace are not comparable run to run. I've been using Training Peaks to schedule training based on training load/stress so we will see if it works (I've got the 80km du Mont Blanc in June, and the CCC in September). Strava does something similar (though without the planning) with their fitness and freshness feature.
Another change is working at hard intensity early in the training phase. Since reading 'Training Essentials for Ultra Running' by Jason Koop which I found pretty useful, the idea is that by working hard on intervals and other high intensity sessions early in the year, rather than building a slow aerobic base early, helps raise your training threshold so that you have a higher ceiling to work within (so the higher your threshold limit, the less relative effort you're putting in to a given speed/effort). I still do long runs, but the emphasis is less on grinding out mile after mile after mile at easy pace and more at improving overall fitness.
Bear in mind I'm not a coach and I'm not you so this could all be b*llocks and feel free to disregard.
What's your goal race in 2018?