5 years ago I did the first 300, or so, miles of the Great Divide trail with a young friend who is almost half my age.
I too was very scared that I would be holding her back, especially with the mountain passes we were going to be riding.

She had taken the summer off and was riding the whole 2493 miles. She and I talked a lot about it before we did the ride, and she said that she would not have any problem riding at my pace; she had a schedule to keep, but it was flexible enough to allow for a couple of extra days if I was slow.

It turned out not to be an issue at all. First, we trained like crazy for the ride; second, she carried about 20 more lbs of supplies than me, (that was a joke with us - her extra 20 lbs equaled my extra 20 years); third, we had a realistic idea of how much ground we wanted to cover each day and rode a steady pace to get there.

There was only one time that my age seemed to affect the ride and that was on the third day when my left knee started acting up. I had to dismount a couple of times that day and walk.. But that was it. Even those high mountain passes were not an issue (and, damn, riding down the other side of those passes was a blast!)

So,tell her what is worrying you and discuss what you both want out of this trip, and go for it!