It really helps to have a trainer or rollers. Besides that, you really only need to ride a total of 100 or so miles/week for maybe a month before the century. That's pretty easy to find time for. You should also get up to 75 miles on one weekend ride, also easy to do, or not, depending on how your training went. If you want to ride a fast century, you'll need more weekly miles that that, but 100 is enough to get it done.