Thanks SO much to all of you. Super helpful. After reading these, I think I will scale back to train for 75 miles, but know that if the terrain is hillier than anticipated, I can bail to the 50 mile length. This particular ride has 25-50-75-100 mile loops. 6+ hours in the seat sounds like too much for me for this year, given the lateness with which I am starting real training, and this is a really scenic ride where I will probably be tempted to stroll along more than is good for a reasonable finish time. There is always next year...based on your replies I think I am certainly capable of a century in the future, just not this season.