I trained for the Etape du Tour this year, and I do a similar amount of commuting to you by the sounds of it. I didn’t really do enough but by getting in some a couple of 2/3rds distance rides and doing some big climbs I managed to build up enough fitness to make it to the end. The advice on nutrition little and often same with fluids, I went mad on food at about 70km in and suffered there after.
Also sound advice above on the descents, they are big and when you are tired can get pretty hairy, I saw quite a few crashes and backed off a bit after hitting 70kms an hour in a few places with people overtaking me and me overtaking people. At that kind of speed braking late, clipping someone or hitting a pothole could really be disasterous.
But my one single biggest bit of advice would be to train your self to do a poo at 5am without fail.