Today I tried to maintain pace and it felt fast to the end but it turns out not. Strava is a good way to compare performance with how you actually felt; it would appear I have 90 minutes of speed in me then it drops off sharply. 

Must do more.

Check out my ride on Strava: https://www.strava.com/activities/1231487919/shareable_images/map_based?hl=en-US&v=1508074291