I have some advice to offer. :-)

Don't get caught up riding someone else's ride. Ride YOUR ride. If someone is going faster than you want to get, let them go ahead and drop back.

Don't go out too hard. It's a sure recipe for disaster 100 miles later. If it doesn't feel "too easy" for the first 20-30 miles, you are probably going too hard. Pacing is key on these long rides.

Start eating early, nibble all day, and don't stop. If you are hungry, and your next planned stop is 10 miles down the road, that's too far away. Grab something out of your pocket immediately, or stop someplace earlier to refuel. Never let yourself get into a deficit. Digging out of that hole takes a long time.

Gear down early and often. Your quads will thank you at mile 125.

Don't stop unless you have to. Progress at 10 mph is better than no progress while taking a break.

You WILL have a low moment. Even the most experienced distance riders do. If you start having negative thoughts, or start getting cranky, it's usually a good sign that you are low on calories. Eat something, take a drink, and focus no further than the next turn on the cue sheet, or the next curve in the road. And keep pedalling.

Recognize that your reflexes and thought processes will slow as you tire. Don't draft someone you don't know in a tired state, take extra care when passing or changing lanes, and don't allow your attention to wander from the road. Remember that everyone else will be getting tired too, and you don't want someone else's bone-headed maneuver to end your ride early.

I don't know what your nutrition looks like, but fully expect that sweet tastes will turn you off eventually. If all you've brought along is traditional energy bars or snacks, at some point those foods will stop appealing. And when you stop eating late in a ride, you won't last long. If you don't have something savory you can pack, try to find a sandwich shop somewhere around mile 100-125. A turkey sandwich around mile 100 sits well in the stomach and the salty savory flavors will make your sweet-fatigued taste buds happy.

Make sure you know how to fix a flat, and have all the tools necessary to take care of yourself.

As you tire, your body loses it's ability to regulate temperatures as easily. Hot weather feels hotter and cool weather feels cooler. Carry sun sleeves and plenty of sunscreen if it looks to be warm. If it looks to be cooler or you will be out late, pack one more layer than you think you need. A vest and armwarmers can be a life saver when you are exhausted and chilly. Even in summer.

That's all I can think of straight off the top of my head, but more will probably come to me later.

I'm off on a 600K (375 mile) brevet this weekend in Alaska (time limit 40 hours.) I'll be putting my own advice to good use. ;-)

Susan