In general, go faster and your speed will increase. The footrest (pedals, in precise terminology) spin around, and the faster they spin, as a rule ....

Intervals: Ride like a maniac for 30-60 seconds, recover until you are just slightly panting, and as soon as you can, do it again, half a dozen times ... barely catch your breath, and do it again.

Also, practice shifting down a gear and spinning faster until you are out of breath/heart rate spikes, alternated with shifting up a gear or two and pushing real hard until your legs won't turn. This develops different muscles and different physical systems.

On days when you don't feel like pushing, just enjoy the ride. Nothing wrong with just enjoying riding a bike. But on some days when you feel okay, keep thinking "I am out here riding to improve," and make sure you Are pushing, all the time, just a little. On those days you might run out of energy a mile or two from home, but the ten-twelve miles before that will work you ought (even beat you up a little) and in the long run you will get stronger.

Then, just keep riding. Your mileage will increase, your strength and speed will increase. it is just physiology. Stressing over it actually slows down the process.

If you are Really motivated ... google all this and you will find bicycle training techniques, in words, picture, and video, in abundance. there are all kinds of exercises and drill and whole training systems out the on the Interwebz, just waiting for you.

Most of all, lighten up and enjoy the ride. Plenty of us have to kill themselves for the kinds of gains you youngsters make just by waking up each day.