You did REALLY well.
Taper usually comes with water gain weight - water binds to carbohydrate.
"Every gram of glycogen is stored with almost three grams of water, which can result in a gain of up to five pounds"
http://www.humankinetics.com/products/showexcerpt....
To take 10 minutes off this result - is totally within realm of reason.
You had a good result. You were disciplined with pacing, which seems to be hard for many guys to manage - so, to do this in a first race is a very good sign.
The Vdot measurement kind of approximates your estimated VO2 max. A first running race is probably not the most accurate measurement, and probably will be less than the truth. However, with your Vdot and your weight, it looks like your ABSOLUTE VO2max (which has to do with size, as well) is probably well over 4L/min.
http://www.runnersweb.com/running/RunnersWeb_Exter...
So, this bodes REALLY well for ironman. For running, the relative VO2max is more relevant. However, for cycling, the abosolute VO2max is more important for cycling on the flats (the majority of an IM course). For climbing, the relative VO2max becomes more important again. Having said that, though - I suspect that your legs are the way they are because you were born with a good measure of fast twitch muscle fibers. If you train hills, you can tap into these - and with continued training, you convert some of the fast twitch into a more aerobic type of fiber.
Ironman has to do with taking what you have and trying to spin it into an advantage.
But, I see some very good signs, here.