If you can average 17 mph you will get round easilly in time.
There arent any signposts. You should have been sent a route sheet or downloaded the route from the site if there is a link (may not be) If your entering on the line then you will get a route sheet when you sign on in the morning .Its not so much navigating as following a set of instructions.
Another way is just to tag on to a bunch of riders, problem is theres no guarantee they know the way and if you get left behind you wont know where you are. Even if you do follow a bunch try to follow the route instructions as you go along.
Its a good idea to take a map of the area incase it really goes belly up .
You get your Brevet Card from the organizer at the start, get it signed / stamped at the control points which may be cafes pubs or someone in a layby on a camping chair.Then hand the completeed card in to the organizer at the finish.
Remember theres no sag wagon to pick you up ,take a couple of spare tubes and some tools.
Enjoy it, once you get into it its a great day out on the bike with like minded people for company.
Which one is it?