Mountain Bike Weight Training

I need some advice on how to train for mountain biking, please?
Im not looking to lose weight, just to get into better shape.
Im 17(female), 105 pounds and around 5’4″, i ride a 18″ specialized hard rock its decent.
I would like to be able to ride for around 15 miles within 2-3 hours but, im only riding 5miles and that takes me around 45 Min’s.
I have only been riding about 2 weeks, 4 times a week.
So if you guys have any tips for me on how to up my endurance, it would be greatly appreciated!
As you already ride 4 times a week and only started 2 weeks ago, you will gradually become more proficient at bike riding. Just be patient. Here are some tips:
To keep up at riding longer distances, you have to actually ride longer distances. You can save the shorter, more vigorous rides for strength and power build-up.
Ride at a moderate pace, but for longer periods to train your aerobic capacity.
Eat and drink to avoid feeling weak while riding – I always choose water and peanut butter (a cheap and nutritious food full of proteins).
Ride with good technique (cornering, descents), as to save speed/energy where possible.
Make sure your bike is set up properly for your body shape – poor posture on a bike can be a real energy waster or cause stress on certain body parts.
Just keep it up and you’ll be fine.
End the Bonk!

