If you've decided to train for an Ironman, hopefully you did enough research to realize that the training is going to be a part time job, that you're not getting paid for, so you have to be quite motivated. If you didn't figure this out before you decided on triathlons, well, SURPRISE! You are now going to have to re-work your life and find 10-20 hours/week for training!
When I started training for my first Ironman, I didn't really have much of a plan. I tried looking up some things online, but most plans were pretty generic. And being the moron that I am, I thought I could develop a better plan on my own. And that plan consisted of riding my bike once or twice a week for as long as I could, running every now and then, and swimming maybe once a week. I was definitely over-training in the "recovery" discipline. My first Ironman, though I finished, wasn't what I had envisioned in my head. I was very, very excited that I was able to cross that finish line, but I was left with the feeling of not having performed to my fullest potential.
Since then, I've read a ton of books, read probably every article on the internet, and even stayed at a couple Holiday Inn's. I am now under the impression that training more frequently is more important than training for longer durations. No, I'm not saying that you don't need to run over 20 miles prior to an Ironman, or that you don't need to bike 100 miles prior to an Ironman. I'm saying that you don't need to do it every week.
For most triathletes, time is very precious. It's much easier to get in 3 or 4 bike workouts, for a shorter duration, rather than going out for 6 hours (and the same is true for running and swimming). Most of us have a hard time finding 6 hours in one day to break free and focus on anything, let alone training. Don't get caught up in thinking that you need to go long every workout, or even every week. Sure, you need to build up to the race distance and actually do the race distances (separately of course) before the actual race. But you don't need to run a marathon every weekend.
By working out more often, your body is able to be in an aerobic zone more often, which will train your body to do all of the confusing chemical stuff that it does to allow you to sustain that aerobic effort. If you only workout a few times per week, you body won't get used to being in that zone. For me, my body seems to react pretty well to training each sport about 3 times per week. Some weeks I get in a little more, and some weeks I get in a little less. Also, by having roughly 9 workouts each week, you only have a few days where you are doubling up on workouts. For the days where you only have one workout, you can either choose to go longer, or raise the intensity level a bit. Do keep in mind that rest is important, and if you feel worn out, you probably are...so rest!
So remember, try and do a few shorter workouts and one longer workout for each sport during the week. If all you do is go long and slow for each workout, you probably won't have a lot of gains.