First off, at the risk of stating the obvious, you have to set your running scheme based on your goals. Just maintain fitness? Do you want speed? To extend your overall long-distance threshold? To complete a specific ultra event?
Let me use my 1998 as an example. I wanted to run the Massanutten Mountain Trails 100-miler. Didn't care about time, just wanted to finish. I also wanted to keep the peace at home by keeping my training as invisible to the family (wife + 2 early teens) as possible. This meant low mileage, and running at creative or even odd times and places (early, early morning; late evening; getting dropped off by my carpool for a run part way home or to work).
So, to prepare I decided that my minimum run would be 10 miles. With only a couple exceptions, whenever I laced up the running shoes I'd cover 10 miles or not go out. For the 6 months prior to MMT, I did these 10-mile minimum weekday runs before work from to , generally twice a week. Family didn't know, didn't care. On the weekends my distance alternated between "short" and "long" weekend runs: 10 on the "short" weekend and 25 on the "long" weekend. To maintain my cloak of invisibility I also did the weekend runs early so I wouldn't shoot half a day.
To get my distance threshold up, I threw in an ultra-distance run every other month (38, 40, and 50). Now, these runs were in fact done during "prime time"--couldn't make them invisible. In 1998, the year I completed MMT, my annual totals were: 1190 miles--107 runs--11.1 miles avg per run. My numbers were real similar for 96 and 97.
I guess I'm trying to make several points:
--Pick your goal and THEN design a training regime around it, BUT.....
--Do your goal picking with your loved ones in mind: no sense alienating those most important to you by embarking on a quest that comes at their unfair expense
--Do be fair to yourself: it is reasonable to plan activities, such as ultrarunning, that benefit no one else, for your own growth/dreams/fitness/etc.
--Scale back: you probably can be a successful ultra runner with low training miles