I did this ride in prep for the Amtrak Century. I did no specific prep for this one except normal mtb'ing and a couple 30-45 mile road rides but those were a few weeks back. In hindsight, I should've done more road as I was pretty spent at the end of the metric due to trying to keep up with guys much faster/stronger than me. I also hike and trail run a bit but prep for longer rides is longer rides in my book.
This was my second 100+ mile ride; the first was in February on the Tour de Palm Springs. While I didn't do anything specific for the Big Ring, I have been riding 70-80 mile/3,000-5,000 foot elev gain rides at least once a month, on both MTB and road bike. Using the MTB for this will make the road bike feel like you're pedaling downhill. The Big Ring took me just over 7 hours (moving time - almost 8 hours including stop lights and water stops). My only goal is to finish. In prep for the Cool Breeze in August, which is 125 miles/7,000 ft climbing, I'm doing Newport Coast and as much of the steep crap in Laguna as I can until I either keel over or my legs stop moving. I don't run, I don't swim, I just ride. Up and down mountains on my MTB, and up and down paved mountains on my road bike. What it comes down to is you should be finding what your current mileage limit is, and increase that by 10% per week, until you're able to do about an 80 mile ride with close to the elevation gain you expect on a century, in a reasonable amount of time. Hope this helps.
