A 15 MPH average on a 50 mile ride is a serious goal, *especially* on a mountain bike/hybrid. I did a 65 mile ride that averaged 16.1 MPH (3,000 ft of elevation gain) this fall and it was really hard and only possible on my road bike, and drafting off of the local collegiate men's cycling team.
I've read that you should focus first on distance and then on speed, though you will naturally improve your speed a little in the process. I followed the oft-cited advice of increasing distance by no more than 10% at a time, and that worked well for me in training for my first century.
On a mountain bike my (hilly) commute is still around 12 MPH, about what it was when I started last winter. But I am a lot less tired and riding a heavier bike than I was then...On a road bike it is a lot faster, and I don't have to work as hard.
A road bike will make a huge difference, and slick tires will help a lot as well.
My biggest piece of advice, though, is not to base your goals on those of others. We are all different, and you are not your daughter. I find that a bunch of realistic short-term goals help motivate me better than a big long-term goal.
Good luck, and most of all, have fun!
Anne
