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I remember reading 60-90min but that was years ago and much more study has been done and this was likely in relation to fat:cho ratios not fat oxidization improvements that everyone craps on about these days.
What racing are you doing? I can understand people trying to improve their fat ox if they are racing ultras but you are doing Vo2s which means you're not training for ultras. So?
If you're only doing an hour there's no benefit to doing them fasted. You just won't be able to train hard enough.