One thing I noticed was that you were getting a little slower as the days progressed. You need to work in some recovery time. A common mistake is trying to do too much too soon. Your performance is very similar to mine when I started. I have now done two centuries, both at about 12.5 average. My fastest average on a 30 mile ride was 16 mph (rolling hills with a net elevation drop of 400 ft.) A few weeks ago I did my flat 4 mile commute at 19 mph, with a little help from the wind.

This took me years to achieve, so be patient, rest as necessary, and keep it fun.

Welcome aboard.