Zoot
The idea is to get your body ready for interval/endurance work. Thats what a lot of riders do. Maybe up to a 1000miles.
But really you need to ask yourself what your ambitions are.

Racing
Sportives
LEJOG
etc etc etc
Racing requires you to ebb and flow with the surges in a race
Sportives a bit more even pace over a distance unless your with the fast ones.
LEJOG, could be as steady as you like for hours and hours.

What are your plans
If your going to race then 15mph at 3 hrs isnt going to cut it. 19/23mph is.
With racing its the surges that hurt you and this is where that endurance comes in. Thats why traditionally chain gangs had the ( If your dropped its tough attitude).