I am in the 90 hour group start. This means I will start sometime between 6pm and 8:30pm. Brest is 616km. At 25kph average that is just over 24 hours riding time. Loose 1 hour per 100km for controlling and eating, call it 30 hours. Coming back will be much slower, I intend to use the entire 90 hours allowed.
Some ifs and buts....
I hope that I will get off before 7pm. 7pm Sunday + 30 hours = 1am Tuesday
I hope that there is no headwind.
I hope that I will be able to hook up with groups going at a perfect speed quite often.
If these conditions are fulfilled then I should be able to make Brest before I turn into a pumpkin. If not then it's Carhaix. If I can leave Carhaix before 9pm Monday then the 93km to Brest should be possible. If not I stop at Carhaix for sleepage. If I have to stop at Carhaix I will have to play it by ear when I leave but it will probably be far too early tuesday AM.
Last time I started at 10:30pm, as noted above the weather was not great and I lost 3 hours due to dropping my brevet card.. But I still made Loudeac. When you make a mistake / have a mechanical early on then it's a double problem as there are no fast groups on the road to get a tow from.
So plan part 1 is to make Brest in 30 hours and then have 4 or 5 hour sleep. Next morning ( that will be Tuesday morning ) I will start to trundle slowly back. For the return, if the first bit of the plan works, I will have a total of 55 hours-ish. So I should have time for another 4 or 5 hours sleep and stopping at Tabacs for the occasional coffee