Good luck on your IM! You're right that it would be silly to underestimate an IM. I trained all year for my Timberman 70.3 and it still took everything I had to finish! I would definitely consider doing at least a couple olympics and maybe a half if you can fit them in. Those 10ks and even a century will give you some idea of what its like, but knowing what it feels like to be at the end of a half marathon and still have half to go is something that even experienced runners have problems with.