I've read enough to be convinced that intervals are a good way to increase my power and speed. But I'm confused on the method. I pedal around 19 MPH, all things being equal (flat surface). When I do my intervals (currently all-out sprints for 15 seconds), afterwards should I continue to try to pedal around 19 mph, or drop down to a lower speed that will allow me to recover more fully before my next interval?