A little late to the party, but I think you should definitely do the 600K. Like others have said, you're not being paid to do the HIM, you're doing them BOTH for the satifaction, pride, and sense of acheivement that you'll get for tackling two major events. Do them both, and ask your coach to please adjust your plan to fit the rides you're doing. My coach adjusts my plan for my 24-hour race to allow for a pesky business trip that hits every year right when my plan should be maxed out. That's what I pay her for.