Get a book from the bookstore or library on bicycle training.

I personally like Chris Carmichael's, but any one will do.

The short answer is that if you just keep riding you will get a little bit faster and be able to ride a lot longer.

In other words, improved endurance is easy, but improved speed is not so easy.

If your goal is to get faster, you will have to train more seriously, that includes intervals/speedwork etc.