My disclaimer: I haven't done Zell Am See, or ridden in that area. Anyway, looking at the course profile it looks like about a 700m climb over about 15km, which is an average grade of about 4.6%. It looks like it kicks up to steeper grades, but it's an Ironman race so it's probably not super steep anywhere.
If your NP in an IM is ~225-240W (ish), how much do you weigh? For climbs, W/kg are important...
You can work out what you'll be comfortable with here: http://www.cyclingpowerlab.com/PowerCadenceCalculator.aspx
Giving it my "sustainable power", weight, etc. it confirms that I should be able to spin comfortably in a 39x28 on up to about a 7% grade, after which I'll need to either up the power or mash. I'd probably ride that course with a 39x27 as my granny gear, but I am light and push about 4W/kg at threshold.
If the numbers look like you won't be able to keep a comfortable cadence on a 39, then consider a compact (or mid-compact) chainset.
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