This decision was made for me by deciding on another goal. I decided to go as fast as I can across America by bike SAG supported. The tour required that we document an average speed of 16.5 MPH on reasonably flat ground and wind neutral (means round trip).
I have pursued this standard for three years now. I met it three years ago but only with extraordinary effort. I did it on a CycleOps FL2 trainer and was sick as a dog after doing 100 miles.
These days I can do better than 16.5 MPH and do 16.5 routinely.
My next tour will be April 2008. I am aiming to be able to do 18 MPH for 100 miles. I am busy training for that now.
BTW, my best time for shorter distances is >22 MPH. We are here talking 100 miles.