Scotty, I try and keep an athlete’s total short term increase in volume to about 20%, and long-term (6 months) to 50%. With that as a guide, the total average weekly volume from the Marathon Level 1 to the 50M Level 1 is exactly 50%. Therefore, the Level 1 is a safe start for you. You can be a bit more aggressive and go to Level 2, but that is a risk/reward balance you’ll have to decide on.
Also, no reason you can’t start with Level 1 and switch to Level 2 after 6-ish weeks. That is a safer method as well.
Regarding the 100K event, I’d recommend one to two weeks “off” (still cross-train, no running) after the 50M and then start up on Week 12 of the 100K plan. That puts you at about 28 weeks of straight training, the upper end of our recommendation, but acceptable.
David