As you are training for back to back rides that is what you need to train for. Ten more total miles each weekend sounds fine. Just listen to your body. If you are still feeling a strain from the weekend on Tuesday slow down. The UltraMarathon Cycling Association has a lot of good words on training for long rides. As you are going for the miles and not some speed record you need to get your body used to being in the saddle for long periods, get the bike set up for comfort, and figure out a good food and water plan. If there is support provided what food and drink they provide might not suit your stomach. Find out what will be provided and see if it works for you.