Just set little goals and realize it takes a while (months/years) for your body to adapt. My commute is 15 miles one-way. I started out riding 1/2 hour in the evenings. Then 45 min. Then 1 hour. Then I drove 1/2 way to work and biked the rest. Then I biked to work 3 days, then 4 then all 5. During all this I drove to my Saturday job (20 miles one way). Then I started biking 1/2 hour on Saturdays, then 1 hour, then biking to my Saturday job as well. There are ups and downs throughout all this as your body gets used to it. For me, this took place over a period of 2 1/2 years.

One thing to think about is that killing yourself by trying to go too fast vs. cutting back to a little more enjoyable and sustainable pace will not be too much of a time difference. Probably 10 min or so.

Also, keep double checking your posture on your bike as you advance. The most important thing I learned is that on a road bike or mtn. bike, many beginners try to sit more upright, thinking that will help their butt not hurt. Putting a bend in your elbows and getting your hips tilted forward helps engage your stronger leg muscles (butt and hams) and actually relieves pressure on your butt.

So just keep increasing your time in the saddle little by little, keep learning about posture, and get enough sleep. It'll be great!