The battery should last around 20 hours so you should have no problem. I have used it for all parts of an IM, and for just the bike/run portions of other races. If you want to only use it on the B/R, then turn it on prior to swimming (set it so it doesn’t auto power off) and place on your bike or in your T1 bag. You can wear the qr wrist strap easily under your wetsuit so its already on your wrist when you get it from your bag.
Jack