Use the Mike Foote Big Vert plan. It is based on the exact training I have used when coaching a number of successful runners for 200+ mile races. You do not need to run crazy miles to be ready for such a race. Have you read this https://uphillathlete.com/ultrarunning-tor-des-geants-luke-nelson/?
Or have you listened to this? https://uphillathlete.com/training-for-ultra-running-podcast/
They might be interesting for you.