Just ride what you got now, the fx is a pretty versatile bike and maybe relatively slow, but if you get some fitness into your legs you should be able to maintain 17-18 on it...especially if you put some clip on aero bars on it. (that's what I did when I had an fx)
changing tires is a good idea too. I suggest Compass Tires, something like the stampede pass or even Grand Bois Cypres. You will probably feel a difference in rolling resistance compared with your current tires.
Compass Bicycles: 700C Tires