An AA alkaline battery's voltage starts at 1.5 and drops to 0.8V cutoff.
This makes alkaline a poor choice for powering an LED. You want a flatter discharge curve. Li-ion is a much better choice. You could easily power these LEDs with a single 18650 battery.
The lifespan is determined by the discharge rate. At 25mA expect 3000 mAH. At 100 mA expect 2500 mAH. See ENERGIZER E91 Specifications The mid point would be 2750 mAH.
Three AA alkaline in series will have a discharge curve from 4.5v to 2.6v. A better value would be 11Ω which would give you 75 mA at the 3.6V mid point between new 4.5V and discharged to 2.77V.
Each one of these LEDs must/should have a current limiting resistor. You measured 76 mA which is only valid for whatever the battery voltage was at that instant. The voltage is continually decreasing as well as current.
First you must specify the current required for the desired brightness. The use an online calculator to get the value of the optimum resistor. I like the Hobby Hour LED Series Resistor Calculator
At 75mA each LED uses 12.5 mA.
A 3.6V 18650 Li-ion will discharge to 3.2V, a much flatter curve. At 4.5V an LED with a Vf of 2.77 would use a 140Ω resistor for 12.5mA. For 12.5mA at the minimum 2.77V the optimum resistor would be 10Ω. The resistor to use would be calculated with the mid point voltage of 3.6V which is 66Ω. But the range of brightness would be noticeable. This will give you an average of 45 mW per LED or 270 mw for all 6 LEDs. With a battery capacity of ≈2750 mAH would yield a lifespan of 36.66 hours with an average 76% efficiency.
If you need a very consistent brightness then a constant current regulator would be required. something like an On-Semi NSI45060JD would work okay. For higher efficiency a switching step down regulator may do better. A TI TPS63030DSKR is very simple, low part count, small, and inexpensive. It would very work well as efficiency would vary between 90% @ 4.5V and 80% at 2.4V. It is designed especially for 2-3 AA series alkaline batteries or a single NiMH or Li-ion.
A switching regulator would yield about 10% more battery life, or about 40 hours.