Well, as some people said, cut her a bit of slack. She did great going that far and that fast on the first day.

I would humbly suggest the following:

1) You want to keep the experience fun and positive or she might give up. So I would emphasize pleasant rides and try to entice her to longer distances by finding rides of that length that are scenic or interesting.

2) Remember this, women have a disadvantage over men in cycling. Men just have more aerobic power. So for a man to get up to 15 mph average is pretty easy. It is harder for a woman and will take a bit more time and conditioning.

3) Another thing about women is they tend to weigh less. I weigh 190 at a pretty lean riding weight. I ride a 20 lb bike (about 10.5% of my weight). Weight isn't critical to me. It is odd that a lot of people put a 120 lb woman on a 28 lb bike (or 23% of her weight) and then are surprised when she can't keep up!!

But 15 mph is something that she should be able to get to without too much time if she does aerobic conditioning most days.