I've read a lot about preparations for long runs, and all articles were accenting weekly mileage as the most important factor to keep form. You could almost get an impression, that all you need is mileage, so simply running 60k a week (eg. split for 6 10k runs) would give you form to run marathon.
It seems strange to me, because fatigue after running 10k in days in row is nothing compared to what come after making 30k in one run... I feel all stabilizing muscles, sore foot etc.
From the ultramarathonists' experience, what is the minimum frequency you need to repeat longest run to keep in form? For example, if your goal are 100k distances, do you need to run 100k at least once a month, or at least once every 2 months, apart from doing weekly mileage?