Using the Nice Ironman 2007 results available online, I've generated simple statistics by the finishing time (by 30 minutes span). This gives me a bit more insight about potential objectives, although I still have no idea how long I'll take.

The following graph represents the number of finishers by time span. It shows a fairly symmetric shape, with most athletes finishing in about 12 hours. This is quite different from many running races such as marathons where the maximum is usually shifted towards the end. This probably means that most athletes are well trained.
ironman: nb finishers by time span

The next graph shows the average split times (in minutes). This has to interpreted with care, as some may be stronger runner, cyclists or swimmers, etc ...
ironman: split times by time span

The following table summarises the average split times for 3 finishing time targets. As expected, the swimming time doesn't show a large variation in absolute value. Thus, I'll simply try to swim efficiently to minimise the overall energy expenditure.
Finishing timeSwimBikeRun
12h +1h106h204h30
14h +1h207h155h20
15h30 +1h258h6h

Ultra speculations.