I've been doing the PMC for a long time (this is year 22 for me). It is incredibly well supported. There are huge rest stops with food, mechanical, medical support at frequent intervals (never more than about 20 miles apart) and vans with mechanical support roaming the course. For repairs, carry what you normally would on a regular ride and you'll be fine. For food/drink, don't overthink it and just rely on the rest stops for your needs. They have water, Gatorade, and a ton of food. For clothing, you can have some pretty big temp swings and weather changes during the course of the day and the weekend so plan accordingly. Pay attention to the forecast and pack appropriately and you'll be fine on that too.

What route are you riding, Sturbridge to Provincetown? If so, the mileage is 110 miles on day 1 and 78 miles on day 2. The stops on day 1 get more frequent through the course and I think they are at mile 24, 42, 69, 85, and 100, or thereabouts. Day 2 I think they are at 22, 40, and 60 miles.

Enjoy it. It's a really fun weekend. With the Sturbridge start you'll find that it's really congested early in the ride due to the number of riders that start there (something like 3,500). Skip the first stop at mile 24 and the crowd will thin out a lot, I've been doing that for as long as I can remember. I usually stop at mile 42 to regroup with some friends but don't spend much time there and the crowd will thin out even further.

You'll be amazed at the event. The amount of volunteers is incredible and you really don't need to do much other than show up, ride your bike and enjoy yourself. The volunteers take care of the rest and they really are the backbone of the entire event, and it owes it's success to their efforts and enthusiasm. The amount of food is also pretty amazing and you can easily overdo it and even gain wait, despite the mileage. Harpoon Brewery is a main sponsor and beer is free and flowing all weekend too! Harpoon even comes out with a new brew every year just for the PMC.