You would expect most top Ironman runners to run in the range of the high 2:20s to mid 2:30s for an open marathon. However, there aren't many data points for study purposes because an open marathon creates too much recovery time with little benefit for a pro Ironman. Nevertheless, Mark Allen, who holds the Hawaii run record in 2:40, once did a run focus period and tried to qualify for the marathon Oly Trials but fell short in his bid to break 2:20. Based on his Carlsbad 5000 time of 14:50-ish, he probably could have run low to mid 2:20s if he was simply going for a best time rather than a qualifier. Ryan Bolton once ran 2:26 as an active pro triathlete with no competition in a small open marathon (Desert Classic in Phoenix, AZ). He was a top collegiate runner before turning to triathlon. Last year or a copule of years ago, Normann Stadler (former Hawaii champ) ran 2:32 at Hamburg or Berlin, but by some accounts it was not a max effort. I have known some second and third tier pros who run in the low-mid 2:30s for an open marathon who typically run 2:55-3:00 for an Ironman split. On the women's side, Joanna Zeiger (2:47) and Desiree Ficker (2:40) have both qualified for Olympic Trials and both have run around 3:00 (Ficker might have been sub-3) in Ironman marathons.
It is also hard to provide a reliable conversion formula largely because Ironman run time is largely a function of bike fitness and bike pacing relative to your fitness. Also, the pro race in Ironmans (especially a deep race like Hawaii) can be fairly tactical with pace changes, so they aren't necessarily time trialing for a best time. Some of the guys who blow up and run close to 3hrs can run faster when they are leading the race against lesser competition. There are also some Ironman athletes without a running background who will run an open marathon as a time trial and simply can't go much faster than their Ironman pace. In Ironman training, the goal is more about how little will you slow down rather than how fast can you go. One reasonably accurate triathlon to marathon conversion is that half ironman run pace will often be similar to open marathon run pace. Its not perfect since many triathlon courses aren't even accurate, but I have found it is a good ballpark measure when the circumstances allow.
