Don't try to push yourself too much just to keep up with your fiance. You will end up hurting yourself. Go with what is comfortable for you. It will especially important for you since you already have bad knees. Until you feel comfortable going fast don't. Have your fiance ride switch. You will prob ride at similar speed.
Keep pushing yourself. Just don't try to keep up with someone well beyond your skill level.
Thanks for the note of caution. I'm definitely not trying to keep with her! Not for another season at least. I want to be able to confidently link turns on greens before trying to link them on blues, but perhaps that is the wrong way to thinkg about it.