How to Structure First 50 Mile Training Plan After Already Doing High Mileage for Years

Hi all! I need some help and know you are where I need to turn. I have been running marathons for years and utilize the Pfitz 18/85 training plan repeatedly. I have essentially run two marathons a year with this 85 mile a week plan repeatedly, so my fitness has been consistently high. I'm achieving my goals in marathons, but I really think I need a new challenge and could thrive in ultras.

I'd like to start with a 50 mile race with a 50k as a training run (as I've seen in many plans). I am basing this off of the fact that I can pretty easily run my 24 mile long runs at close to marathon pace, so I think I'm already in shape for a 50k. I have searched reddit (thank you by the way!) and already bought and read all of the beginner ultra books that were recommended. However, the plans included are significantly less mileage than I'm already used to. For this reason, I'm unsure how to structure a training plan other than moving some runs around to have two long runs back to back. My biggest learning curve will obviously be getting used to trails rather than roads, which is definitely intimidating.

Is there anywhere else I should look for plans or are there any other suggestions you have for me? I would greatly appreciate it!!