I had never even done a sprint triathlon before but decided I'd do an Ironman to test myself - so entered the UK Ironman 2012 and gave myself 20 weeks to train for it. I didn't exactly set the world alight, but got around in 15 hours flat.
It is worth noting that in a survey of 50,000 athletes, Ironman UK was considered a lot tougher than others such as Austria or Switzerland. Of course this is opinion not fact - something to be taken into acccount.
I am pretty active and have done one marathon (4 hours), a handful of Etapes but almost anyone can do an Ironman if they (a) stick to the plan and (b) stick to the plan ! As others have pointed out, the training is time consuming although you don't need to become a complete hermit, just an occasional recluse.
Lots of people will tell you are mad/can't be done etc etc - don't pay any attenton. It was an amazing experience and there is no feeling quite like all that hard work paying off.
One tip I would give, is to try and pal up with someone else who has entered, as training along for 10-12 hours a week is tough on the mind. It is good prepartion for the race itself though, no drafting on the bikes means you are alone until the run.