The most important thing about getting swim fit for an Ironman-distance triathlon is to spend time in the water regularly and consistently. Twice a week is ideal if you can do it on a regular basis.
Get your swim stroke looked at – it’s the best way to make improvements in your swimming and make your training, and your race, easier. When you know what you need to work on to make your stroke easier and more effective you’ll have drills in your locker that you can refer back to and help keep your swimming on track.
All your training swims should have some sort of technical emphasis to them. That’s not to say that you need to do lots of technical swim sessions. Rather, during your warm-ups think about what you’re going to focus on that day. In steadier, aerobic sessions you could combine drills into the session, e.g. in a set of 20 x 100m, do the first 25m of each 100m as a drill.
Other than the technical focus, training for distance swimming is no different really to training for longer bikes or runs. Of course, the distances and times spent in the water will change, but the principles are exactly the same. The main thing I suggest to anyone going long, whether swimming or across triathlon as a whole, is to make sure that you vary your efforts. This will become even more important when you come to swim and train in open water. If you stick to one pace, you’ll only have one pace, and it’ll make getting fitter or swimming further in one go a far more laborious process.
If you train, however, the same way as you would train for your marathon runs – alternating longer steadier swims with shorter faster sprint intervals, and adding tempo/threshold/CSS efforts in between – you’ll get more enjoyment from your training and a faster physiological response.
On the basis that you might swim twice a week, you could do a longer, more aerobic swim once a week, and alternate the other session between shorter, faster sprint efforts and a session predominantly using tempo efforts. When the spring comes around, you might want to do fewer of the sprint sessions and concentrate on the threshold swims, as well as doing more of these training sessions in the open water.
Finally, as you near race day you could take the threshold efforts out and focus on the sprint speed again, to keep your body and form feeling sharp. Good luck!