You are already running, riding and swimming well in excess of the distances necessary for your HIM and you are still 6 weeks out. I don't think two weeks of altered plans is going to greatly effect your performance at your HIM. You will still make your goal times, I'm sure of it.

I think you should do the 600K. In the big picture, isn't it a pretty major stepping stone for you? I don't think you should pass up this opportunity.