Based on those run times then in theory 90 minutes should be comfortable, but obviously you haven't had a chance to build up running endurance, which makes it more interesting. Best way of keeping the legs fresh is to remember it's a 90k bike and not start at your Olympic pace and try to hang on. You'll probably feel it's very easy early on and want to push more, but you'll get the benefit later.
As for the helmet, stick with the normal one. It's supposed to be a hot weekend and you'll be a lot cooler, with no worries about it getting uncomfortable. Plus, if you drift outside your target time you can blame the helmet. 