Humbug. If you find that you only have an hour to ride in the evening and you find that your idea of fun is smashing yourself into oblivion and pulling yourself inside out for no other reason that just because you can, then have at it. That's one of the fun things about bikes - one day you could be cruising along enjoying the sights and taking in the fresh air, the next you're inhaling air like it's going out of fashion as you smash a gear you never thought you could manage up a hill before flying down the otherside...
As KD said - you need to focus on both speed and distance. Distance isn't that hard to train for once you've worked out the ****les centered around comfort. Saddle, shoes, shorts, stem - figure out where the saddle needs to be, the correct length and position for your bars (via the correct stem) and get a pair of shoes and shorts that fit very well and you'll be most of the way there.
For the first month or so just go out and ride. The more you ride the better you will get.
Training involves a progressive increase in load - ie you gradually increase your training but not to a point where you're constantly overly fatigued. It's ok to feel sore one or two days a week but not every day week in and week out.
A lot of cyclists use the weekend to get their long rides in. Start off by figuring out how far you can ride without feeling like death on the bike. Remember to drink often (a standard sized bottle per 60 to 80 minutes in coolish weather) and eat something or use a good sports drink. Use that as your baseline "long ride"
Once you get a little more comfortable riding for an hour or so set aside a couple of rides where you split and hour up into two or three sections (intervals) and ride each interval a little faster than you could ride for the whole hour. Start of at say 15 minutes and ride a bit harder then ride easy for 5 minutes. Use the first couple of sessions to gauge how hard you can ride. Don't overly force it and it's probably better to go a little easy at first but if you do run out of steam after 5 minutes then no biggie - back off and rest for 5 then try another 15 at a more suitable pace.
You don't need a fancy training program but you'll probably need a plan. A plan for a goal event, like your century, makes things so much easier. Even something as simple as "ride 5 miles further on the saturday and sunday rides each week" is a reasonably attainable and defined goal that you can mark down on a calendar well in advance and work towards that goal.
Books like The Cyclists Training Bible make a good read. Your local library probably has a copy...