"Very flat" makes it easier to complete a distance you're not used to... IF it isn't also "very windy." The #1 thing that distinguishes cycling from running is that on a flat course, you can slow down and cruise.

What's your longest ride to date? What's the longest you've been doing regularly? I think barring adverse weather, if you've been riding 30 regularly, you should be fine at 62 if you just take it easy and don't try to stay with faster riders early on.