I am in the same boat as many. I have 5 centuries under my belt. The last 2 yrs the longest ride were metrics. That is the perfect distance for me where I have to push a bit from mile 50 but don't feel knocked out at the end. It is mile 75 where I have had more than enough time on the bike.
This year I signed up for yet again a century Ride to Montauk since the way it is set up it is too inconvenient to ride the metric. However, for the first time in years I am excited about it. I completed my sub challenge and rode the bike 100 consecutive days (full challenge is 300 for the year no less than 10 miles since some rides are on the trainer). I feel stronger and hope to do the century in less time.
Like Marni said, part is mental and I have it in my head that it is a day to be on the bike and stopping to take a break every 25 miles. After that 75th mile I will stop to stretch and enjoy the scenery if need be. Now lets hope it works.
2012 Specialized Amira S-Works
2012 Vita Elite
2011 Specialized Dolce Elite (raffle prize) - Riva Road 155
Ralaigh Tara Mtn Bike