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just to add to the advice re:
"the inefficiency of wearing trainers on the bike for more than an hour will cost you more time than the 30 seconds "
if its a very short first timer tri then you will only be on the bike for maybe 25 minutes and then theres not so much of a case to change shoes - even if you are used to riding clipless, thats what i think - but im often wrong, so the missus says