My top tips;
Set off after half an hour; you get people to pass, and will get caught by faster moving groups so you can tag along if you want to.
Aim to do a negative split; ride the first half slower than the second.
Clothing is weather dependent, but try to avoid big blowy waterproofs and chunky overshoes unless you know the weather will be bad enough to justify them.
Arm warmers and a windproof gilet can be removed if you heat up, similarly, go for thin lightweight gloves that you can stick in a pocket.
My preference is for oversocks; they keep the worst of the dirt and grit off your shoes without turning into huge soggy neoprene wellies.
Eat and drink to fuel your riding for hours ahead; what you eat in the first third will fuel you for the second third and so on.
If you are used to gels, you can use them plus energy drink.
Although you aren't going for a time, stopping for any more than a few minutes will make you feel worse.
Above all it's a mind game; know where the half way point is, but don't keep looking at your computer.
Do mini stretches to avoid getting stiff in your back or neck, and change hand positions often.
Try to ride one gear easier than your mates; spinning is important.
Take a CO2 cartridge and a pump; if you puncture, get the tyre on an properly seated using the pump, then use the cartridge; easy to run out of enthusiasm and mojo when you flat.
Use your brains; don't be the donkey for your group, don't sit out in the wind too much, take care when riding closely with others, especially at the end.