We did something similar at my work to raise money for charity, I peaked at 548 miles in one week, that was 32 hours on the bike, lot of early morning rides before work and then doubling in the evening. The key is to go
EASY, zero tempo efforts or anything even remotely hard. I kept my elevation to a minimum, as much as I could in this area, and kept the effort easy on every ride, no sprints, no going after segments, etc..
As mentioned, the other key is sleep and fueling, have to get as much rest as you can, and make sure you're eating enough to fuel the effort. I ate everything that wasn't nailed down and still dropped 5lbs through the seven week challenge.
If you have time before the challenge starts, then def start ramping up your volume. But most importantly, keep all the efforts easy, then it's just investing the time.