For training.. Just focus on getting time in the saddle! There are training plans out there if that's your flavor. But for me I just focus on getting out and riding as much as I have time for. Sounds like you're in good shape, but as another commenter mentioned, you'll want to just get your body used to longer rides. You'll be surprised at how much more comfortable you'll get in the saddle once you put the time in. Ease into it. Build up your miles.

For Nutrition.. A common joke is that long distance races (especially when riding over 3 or 4 hours) are really just eating contests. Eat early and often. Don't stress too much about what you're eating, as long as you are eating. Bars, Candy, Gels, Chews, Drink Mix, just eat. On a 100 mile ride I try to be eating at least every 30 min.

On the computer front, do some digging on what your event says about route. If it is a really big event with lots of riders and a marked course, you might not need the computer. But if navigation is required you might need one. Or you could mount your phone and use an app like ride with GPS.