Cycling is a multi year sport. It can take years to really build up. So you are off to a great start!

To get speed you need a strong foundation of lots and lots and lots of base miles. Keep riding and you'll build endurance. Once that's established far later this year or next season add hills, interval work, maybe lift weights.

But without a solid foundation of base miles all that's for naught. So keep riding. Speed will come.

Most posting 15 mph+ are on road bikes and not on dirt. So you are doing great. Slicks would not hurt (smoother tires if you have knobbies on the comfort bike) but really ....

8 mph on dirt with a comfort bike?

You rock!