1. Get a bigger motor
2. Get more aero
Other factors are likely second order, but important for pros.

Wind resistance increases as the square of "airspeed" so wind resistance at 19 mph is 60% greater than at 15mph. the really bad news is that required power (Power = Force x Velocity) increases as the cube of velocity (zero wind, flat). So ignoring second order effects and assuming you're already aero, you'll have to bump up your power by a factor of 2.

There is some good news. If your average is over a hilly course your average is being greatly affected by your climbing speed. Climbing occurs at a lower speed and the Power required approaches a linear function of velocity.