First off good luck!
Take heed of all the wise words about liquids and calories. You'll be using the equivalent of a couple of DAYS of regular calorie intake. On anything over 50 miles or if it's hot, I'll usually take at least one bottle of proper powder mix energy drink (SIS, High5 etc etc) instead of plain water to give that extra bit of energy and help with any cramp. Drink early and often, not just when you start to feel thirsty, even if it's cool or raining.
Next, you might feel a little self conscious about Lycra, but as well as a bit of padding you'll find that anything baggy or loose around the groin could cause chafing. Chafing is bad. Also, consider getting a pair of padded cycling gloves. Although you'll have the benefit of front suspension, it's surprising how much vibration and pressure your hands and wrists have to deal with on a long ride.
150 riders in total doesn't sound like a big event, so I' guessing mechanical support on route will be limited. Even if it's good, remember to take a couple of spare inner tubes (you can take a puncture repair kit as a final backup, but they're faff on a ride) and some decent tyre levers. If you haven't changed a tube in a while, find a vid on YouTube and have a quick practice. A cycle multitool/chain tool might be useful, but only if you're confident in making a few basic repairs.
Although it sounds like I've given you a shopping list, if you've got a
Decathlon nearby you could pick up serviceable kit for not very much, or go online to big cycle sites like wiggle or
chain reaction and check out their own brand stuff. A decent local bike shop (LBS) should give you decent advice and let you try stuff on, but is likely to be more expensive.
After all that, don't overload with rucksacks and crap, especially on a lumpy ride. Watch the weather forecasts from a few days before to get a feel for the weather so you don't end up carrying or wearing too much on the day - this is more common than you'd believe. Buy, borrow or steal a saddle bag for what you need and make your supporters have the rest ready when you need it at the stops.
Sorry - a bit of an essay! Make sure you let us know how you get on.
Grats.
