How to build stamina and speed? I am a 28-year-old female weighing 13 stone [approximately 180 lbs] (overweight, I know, as I am only 5'1"). I've recently bought a mountain bike to ride to and from work (about 2 miles each way), but I'm struggling to actually do the whole journey on the bike, as I have to get off and walk. I also want to be able to do this journey in 15 minutes. I am asthmatic, but since I started riding a week ago, I'm finding I'm not as out of breath already (good start).

I'm just wondering how to build up strength, stamina and speed, as I am currently struggling on the smallest of inclines.



Thank you for all your answers/comments.  They have all made me feel a bit better.  I am going to go to Halfords on Tuesday to check everything is right height etc. The gears I'm struggling with currently but sure I'll find the right ones soon... hoping eventually I can get to work in around 15 mins.  I have started making sure I drink water about 500mL (1 pint) about 30 mins before I ride and start stretching my legs.  I am determined to conquer the hills going to and from work setting myself goals as I go up them.
You're doing it already. Time on the bike is what counts the most. 

In the early days you will find yourself pushing up a lot of hills but in a few weeks you'll be surprised at how much better you're doing. When you feel like a ride is going OK, try to get a little further than before on the uphill bits - maybe an extra lamp post. That might be on a Monday after a weekend rest (or it might not). If you're feeling tired, don't stress,walk when you have to .

You'll build cycling muscles, which will make you more efficient as well as stronger. And your lung capacity will improve - it sounds like this is considerably more exercise than you used to do. Then, all else being equal, the weight will start to come down a little. I suggest you measure your progress - plenty of decent bike computer apps are free (assuming you've got a smartphone).