runnersworld.com has a nice calculator for such things.

http://www.runnersworld.com/...1-51-58-1002,00.html

It's moderately accurate, but assumes that you are in proper shape for the event you are about to do.

but anyway this is what I've done

5k 20

10k 42 at alt when I lived at sea

1/2 1.30-1.33

marathon no idea

Oly tri 42-44 both times I've done them

IM Marathon 1/2 way in 1.59 all the way in 4.55, no cramps, no nutritional issues, simply could not keep head in game and did not want to run.

Those numbers are simliar to mine.

Never done a standalone 10k but have done some 35min 10k's in an OD tri. My PB for 1/2 marathon is 1:16 which was done early in the year.

Standalone marathon PB is 2:43 (Boston) and best IM marathon is 3:15.

Those numbers seem to be typical of some of my training buddies as well.

A few points in no particular order of importance:

1. The running portion of a triathlon is a unique event/activity that is indirectly related to ones pure running talent and fitness. The key feature being the ability to run fast when already in a fatigued state from the efforts of swimming and cycling.

2. Having good stand-alone running times is a great place to start. I would say that this pre-disposes one to perhaps being able to run well off the bike, but this is not assured. Like all factors of endurance sport this is something that has to be worked on. I have known 2:15 marathoners who have completely blown up on the run in 1/2 and full Ironman races

3. Run courses like swim and bike courses in many triathlons are notoriously inaccurate. To my knowledge there is little if any sanctioning process in the sport to ensure that a "10K run" in a triathlon is a 10K run and not 9.2K. In running events beyond a certain size, the course must be measured with a fair degree of accuracy to get it's certification and sanction even at the local level.

4. At the Ironman distance there seems to be some acceptence that all things being equal( and this is very hard to do) that if you are optimizing your run in an IM it should be about 20 - 30 minutes slower than your stand-alone marathon time. For me this works( 2:40/3:00) and the straight math seems to work at the 1/2 Iron distance with a 10 minute differential as well(1:13/1:23) and then droping to an Olympic distance race with 5 minutes differential(31/36).

5. As a follow-up to #3, I recall ran much faster run splits many triathlon races, but I am very suspect of the accuracy of those run courses. Example: I recall a few of 32 min "10k's"

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Steve Fleck @stevefleck | Blog

running wise these times fit perfectly with the normal "curve"

10km time x 2 + 10' = 1/2m time (suggest a 33'+ 10km which makes sense with a 35' 10km off the bike)

then

half marathon time x2 +10' = marathon time
only 1' off.

then converting these times in tri is virtually impossible. Looking at elite times over 10km and times in a marathon IM and there are big discrepancies.