I understand the both the pressure of feeling like you have to keep up with someone and the guilt from holding them back. I prefer to avoid that kind of pressure/guilt. I have ridden by myself because of it.

However, having said that, I have done some rides with my "very strong" husband, who has at times made the decision to ride my pace with me and pull me. This is fairly rare as he loves to push his limits and be one of the first one's across the finish line. But when he makes this decision, I have no guilt. In your case, it sounds like your friend isn't quite as easy going, so I'd just avoid the situation. As many have said, you're likely to find others going your pace that you can hang with.

For general riding purposes, I do encourage you to try to find a new riding partner(s) that does go your pace. Through a cycling club or wherever you can. I have a riding buddy who makes getting out there a a lot more fun and I ride a lot more often because of her. She is stronger than me on the hills, so she either waits for me at the top or I pass her on the way down (depending on the hill). Yet she also inspires me to ride better because I try to stay with her on the hills whenever possible, and sometimes I can. We ride the flats about the same.

So my advice is:

1) Don't ride with your "friend" if it will stress you out.
2) See if you can find people at the event that are riding your pace.
3) Do try to find a "different" riding partner or partners that are closer to your level. If you can do this before your event, then great. If not, maybe you'll find a partner before the next one and simply to train with.

Good luck on your century! My advice on that ... focus on the rest stops. Don't think of it as riding 100 miles. Think of it as riding 23 to the next rest stop (or however many miles it is). Prepare as best you can and HAVE FUN!!!!!!!!