What was your training like for the 50k? I'm doing the 100k L2B in June and have started running 4 times a week with 2-3 cross training sessions. After a few months and I've built my base fitness up, I plan to drop one of the XC sessions and put in two long runs every other weekend. I'll slowly build that up to the 25-35mile point I think.
From what I've read, its quite common to try and do back to back long runs on weekends, they get your legs used to moving when tired and there's another whole piece on the body working on low glycogen stores over those longer days as well. There's quite a few ultramarathon guides out there that are worth reading.