Unfortunately, if she's already feeling left out and insisting that you all 3 have to do this together, it's going to be hard for you to separate your training from her without damaging your friendship, and you may end up having to do that. Do you really want to ride the entire century at her pace or at your own pace?


The recovery ride suggestion is a good one.

If your friend'll agree to it, about midway through the recovery ride with her, start having her practice drafting - that way you can pull her for a ways and challenge yourself for a bit. You and your other stronger friend can take turns pulling and just treat it as a way to prepare for the century - drafting each other for a short time to take a break.

Another thing you could do is just tell her that you will match her pace on the flats, but you really want to push yourself on hills... And just treat the hills as intervals where you ride as hard as you can to get to the top, and then just wait for her.