THank you for the responses so far!

I would say I am in pretty decent/good shape. Just last week I reached my weightloss goal of 38 pounds! (been trying since January 2007). I have good cardio capacity, I race half marathons and shorter distances, have done 1 marathon, work out 5 to 6 times a week.

But cycling is so new to me and I feel slightly unsteady. I've ridden bikes before, last long ride was a mountain bike ride in about 2006.

Should I do it by time and not mileage? Aim for a certain cadence?