As @willball12 says, nutrition and hydration are key for a double century, as is pacing. Work out in advance what food works for you, and remember to eat and drink regularly - much easier to keep on top of things than try and catch up. Take climbs steadily and try not to go into the red too much - on long rides like this it can take a long time to recover from that kind of effort, you need to go at your own pace.
The Century plan will see you right in terms of fitness, but IMO it’s worth doing some longer outdoor rides in order to prepare yourself properly mentally for the distance and to make sure you are comfortable on the bike for longer periods.
You don’t say how much experience of long rides you have. If you haven’t done many centuries then I’d recommend getting a few under your belt before the event. I did a 210 mile ride a couple of years ago and in the year before I did a century each month, extending this to a 145 mile ride and a 190 mile ride in the two months before the event. The 190 mile was probably unnecessary but I wanted to do a near-distance ride at a slightly more relaxed pace to get an idea of how hard I could push.
I find it best to try and ignore the GPS as much as possible until very late in the ride - seeing that you’ve got 100 miles to go when you’ve already been on the bike for several hours can be very demotivating. I set the main screen of my GPS to only show time - this meant I could keep a check on when I should be eating and drinking, but wasn’t distracted by other things. You will almost certainly question yourself and want to quit at some point in the ride, but just keep pedalling and it will pass.
My fuelling strategy is generally to drink at least one bottle per hour, the first bottle containing SIS GO carb drink, the remaining containing electrolytes only. After the first hour I start with solid foods (usually SIS or High5 bars - one small bar or half a large bar every half hour), then move on to gels for the last part of the ride (again, SIS or High5 work for me - one gel every 20 minutes). I incorporate caffeine into my nutrition at key points in the ride, either via gels or electrolyte drink, but it is key to work out how you respond to caffeine in your preparatory rides.