How slow is slow? 20 kph rolling average is fine. On my final PBP I averaged 20.7 kph, and I was 68 at the time. My downfall (I was 3 hours late at the finish) was reckoning I wouldn't make it and catching a 3-hour zone-out peopled with coffee & croissants at Mortagne.
Tell you what: why not plan a 200k and keep your average speed down to 20-21 kph? Once you finish, ask yourself if you could do another 100k. Riding slowly and steadily works wonders.
Anyway, the positive? The 12-year-old at a stall in the depths of Normandy who greets you in the rain back of midnight with "Hello, my name is Thomas, do you speak English? Would you like a coffee?" Or the crazy lady with bows and streamers in her hair who is there at 2 am with several kids serving you coffee, water, biscuits and precious sit-downs at 2 am. Or finding country shops that are staying open all night to serve you, and that will be happy to microwave a pie for you. All happily greeting you, all helpful. Or catching the first view of Brest from the top of the hill down to the Pont Albert-Louppe and stopping halfway across to take pictures.
Or just knowing that you're on the road with 6000 other cyclists, most of them suffering just as much as you.
It's all just bloody wonderful, so please don't miss it.
Oh, and not to devalue your partner's readiness to support you, but a support vehicle carries with it a great temptation to quit. One of the tips to BRM organizers is not to plan figure-of-eight circuits, because the temptation to cut it short halfway will be too much for some. Also, a chum's wife supported him all the way round PBP 1999 and hated it.
