I did my first century after 5 months training. I had never ridden before.

I often cannot ride outside during the week, but find that spinning or riding a trainer indoors during the week allows you to maintain your fitness between long rides. So don't give up too quickly on the idea of a full century just because you can only do long rides on the weekends.

If you can ride 75 - 85 miles in a day you can do a century, as long as you're riding comparable terrain to the century you're going to do.