And if your goal is get fit, then there are a lot better ways to do it than measure your average speed, and certainly much better ways than measuring your average speed against others, riding different terrain under different circumstances.

Focusing on average speed, is actually counterproductive. (use the search function to find the repeated voluminous explanations of why this is so.)

If you're doing this to get as fit as possible, and don't care where you stack up against others, follow a training program based on time and intensity, not speed, and use an HRM, or a power meter if it's in your budget.

If you're doing it to be competitive find competitive group rides or races. In either case, Average speed is not only the wrong matrix, it's often counterproductive.