Indy has pointed you in the direction of a very useful thread, Liz.

Remember that training/everyday averages should not be your fastest pace. That means most of your riding should be at a slower speed than your fastest possible speed.

If you rode at your fastest/hardest effort every or most rides, you will wear yourself out and your fitness would actually go backwards.

If you want to measure improvements, then choose one course and measure your speed on it every 3-4 weeks...

There is a lot more detail in the other thread