Well, Dana, my dear, you have signed yourself up to ride 100 miles in one day. That's what a century means in bicycling terms. There are also half-centuries (50 miles) and metric centuries (approx. 62-65 miles). But a no-kidding century is the full 100 miles.
I must admit that my longest yet is the metric, but loads of people do centuries all the time, so while they're challenging, they're not exactly superhuman.
Get hold of "A Woman's Guide to Cycling" by Susan Weaver--she offers some training schedules for riders preparing to do a century. Most of all, HAVE FUN!
--J
"Do I contradict myself? Very well, then, I contradict myself." --Walt Whitman