I couldn't do it now, but back in high school I did the Cleveland-Revco marathon. Yes, I was a Clydesdale even back then; at my height it's tough not to be.
Finished in just under 3:45:00.
It can be done. It's not easy. Your training seems sound. Best of luck with it.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.