Did Pedal to Paris myself back in 2010. It's not a hard ride, really. Day one is the toughest, but mainly because of the time pressure- there's a ferry, it's booked, they want you all on it. But the Legion's got this ride down to a fine art. You'll have a rolling roadblock to get you all out of London, and you will fly along. Yes, you, no matter how slow you claim you are. It's like being in a Grand Tour peloton, only a bit bigger and a bit slower
You will be well fed, and if you have a mechanical there will be help. If you need it, there will be a broom wagon. And you most definitely will not get lost. In other words, stop worrying and ride your bike. Then ride it some more. Any and all riding you do in the meantime will help.
My write-up here:
https://www.cyclechat.net/threads/royal-british-legion-pedal-to-paris-2010.61518/#post-1153912