Speed is a byproduct of watts per kilo.... if you have too many kilos or not enough watts or incline 17mph might be out of reach. Speed is also hindered by aerodynamic drag and your ability to get aero. Frankly having a goal of an average speed for an hour is a good goal, how realistic will depend on you and where you set it. As #Seattle Forrest hinted at flat is something I can't really relate to as most of my flat rides are 2% and into the wind half the time.

Build your base, include long rides. Once you have a solid base add intensity and distance. Rest and recovery will play as important of a role as hard days.