I’ve ran a 42.5 mile Ultra and am running a 70 mile Ultra booked in for June (Wall Ultra).
I am probably not the best person to give advice, as I’ve never really followed a plan or researched one properly, but my week is typically a couple of 10ks, a threshold type session, and I’m just moving into two long runs (back to back) over the weekend. Last week I popped out a 26.5 miles and 30 miles but that will be the longest I will go. Moving forward it will be a Marathon most weekends and then back it up with a half, 15 miles the following day
One of my friends runs 100 milers and he runs no further than that in training but really focuses on the weight training.
I ran Coniston Marathon last year. That was particularly tough, so a great race if you want to push your boundaries and the race is incredibly beautiful.