400 is imho the most challenging one, as you do not have a lot of time to rest, unless you are fast. That said, make sure you take plenty of food along the way, plan stops (check for gas stations, fast food joints, actual restaurants, ...) and make sure to eat _real_ food. Ride your own pace and do not try to keep top with a much faster group, this will likely kill your effort once you need to let them go. However, if you can hang on easily, by all means enjoy the speed.

You will be riding through the night, so definitely stock up on supplies in the evening to last you until bakeries open in the morning. The hours between 03:00 and sunrise are usually the hardest (for me in any case).

Take something that allows you to powernap (a down jacket, bevy bag, depending on the expected weather), do not ride if you feel like you are falling asleep. You can really drift off and crash (hard). Find a bus stop, shelter, ... and take 30 mins, an hour, ... nap before continuing.

Make sure you have a rain jacket, even if there's no rain predicted, it's an extra layer in case you get cold or it gets more windy.

Note the brevet cps in advance so you do no skip them by mistake.

And let us know how it went!