You should aim for the speed at which you're happy.

Some people like to go as fast as they can, and are competitive, either against themselves or others, even if it means ignoring lots of pain along the way. Some people just like to cruise along and enjoy the outdoors and don't care who's passing them. Most are probably somewhere in between.

I think what matters most in terms of average speed as well as ride distance is enjoying yourself and having fun, whatever your definition of fun is.