What are your eventual goals?
To most beginners 20 miles seems like a lot. My local club does not have any rides less than 24 miles (only one that short).
Still 6 miles is just fine. But I'd say it is not a 'stable' distance to ride. I'd say that within a year the odds are either that you will consider 25 miles a rather ordinary (or short) ride or you won't be riding much.
I really do not want to be 'scary', rather encouraging. Cycling seems to be something where one rather quickly finds they can look back and wonder why they thought a ride was hard.