I think the POE injectors are "intelligent" and supply the amount of power needed. So a short run would not require much or any, and I think the POE injector would not "oversupply" power. Somebody else please confirm or deny.
It's not like phantom power that hits 48v all the time.
The POE supplies a constant voltage and then devices downstream draw what ever current they need. Not so much intelligent but just the way voltage and current work. If you have a power supply that puts out 12VDC and can supply 20 amps of total current and you hook up a device that needs 12VDC and only draws 1 amp the other 19 amps of available current just sits there unused. It's actually very much like 48V phantom power. I suppose different mics draws different amounts of current depending on their size and circuitry.