I used a 2-AA LED 3-watt mag lite on my road bike and it worked fine. Small, light weight, and 2.5 hour run time with just 2 AA disposable Alkaline batteries. I did an all night run once with it and it worked out fine. Check-in and rest stops were about 2 hours apart so I would always put in fresh batteries at every stop. I later modified the light to run with a bottle type dynamo. I used 2 junk Ni-CAD batteries externally mounted between the light and the bridge rectifier used with the dynamo to convert AC to DC. The batteries had very little capacity left in them and would only power the flashlight for about 15 minutes. Normally I would have thrown them in a battery recycling box at Radio Shack. I used them as voltage and current regulators for the mag lite flashlight. The batteries load the dynamo when output voltage approaches the rated voltage of the batteries. This prevents voltage from exceeding about 3 volts. The batteries also absorb excess current not used by the light. As the batteries were worn out junk overcharging them was not a problem. There efficiency was so poor past there intended usefulness the excess current simply vanished in the form of heat. You could fill them getting a little warm but nothing to be worried about. The batteries also kept the flashlight illuminated for about 15 minutes at stops. I cut a fiberglass tent pole to use as a replacement for the batteries in the flashlight. It already had a hole in the center so a wire could be easily pushed through and soldered to a modified wire terminal for the positive battery contact. The other end had another wire terminal soldered to the ground wire with a second terminal acting as a washer so the spring in the battery cap would not bind up as you screw in down. I had to drill a small hole in the middle of the cap to pass the wire through. Here is a photo of the modified light.
It would be very easy to attach the light to a 2 D battery holder. 2 D batteries would easily power the Mag light all night.
Here is a photo of it mounted on the handlebar of my touring bike.
I used this lighting setup until I built a new headlight using a Luxeon 1-watt LED, matching Fraen elliptical optics, all mounted in a 1” PVC pipe coupling.
http://www.luxeonstar.com/
The reason for the upgrade was the ability to power and recharge a Palm TIX and GPS receiver while on tour. Here is the circuit I use.
The 4 Ni-MH regulate both voltage and allow excess current not used by the LED headlight and taillight to be used as recharging current when riding at night. They also set the proper voltage to meet USB power requirements. During daylight hours the lights are turned off with S1 and a USB cable pluged into the USB port allows operating and recharging the Palm TIX and a bluetooth GPS unit. The capacitor across the headlight is not necessary when the batteries are in the circuit but I included it so if my batteries were lost, damaged, or stolen I could still use the light with the dynamo.