My husband and I just did a century. I think I would call it "training" that we did all summer, but oddly enough we didn't have a particular mileage goal other than doing at least 20, 2 or 3 evenings a week and a longish one on the weekends, 40 or so. Sometimes we did a longish one on both weekend days. I think for us, getting in the saddle as often as possible was helpful. Also, seeking out hills on purpose.