Forget averages... while interesting to look at, they do not take account of head or tail winds, or hills and should not be how you measure your overall performance

Train according to time; do not make every ride a fast one - long, slower paced rides (eg, 90 minutes at 65% if you have a heart rate monitor), and slow active-recovery rides are vital in your training programme. Time you set yourself will depend on the type of riding you do.

Have a couple of courses you do that you can record PBs on... eg an 18km flat course, or a 25km hilly course. Record weather conditions and try to ride that course on similar days/conditions. I find that the best way to measure personal improvements.