Every time I turn on a regular flashlight all I keep thinking is, "Hurry up, turn it off, the batteries will wear out."

I don't like that.  I want life to be more relaxed.

This is the LED flashlight I made.  It's a convenient size and it works as both a regular bright flashlight, and a long lasting LED flashlight:
 

Features:

1. Normal Lumilite 2 C battery flashlight with krypton bulb that draws 700 mA.  $4.49 at Target. (note:  2 C flashlights are hard to find.)

 
2. Change the bulb and add a resistor plate to get an LED flashlight that draws about 20 mA.  It uses a red LED at 5000 mcd, or a yellow LED at 23,000 mcd.  Battery life is proportional to current drain rate, so a 20 mA light will last about 700 / 20 = 35 times longer.

 
3. The red one is good for the middle of the night when night vision is active.  It's enough light to see a few things and your eyes won't have to adjust to a brighter light level when you turn it on.  It also works well for walking at night because other people can see it from far away.

 
4. New alkaline batteries are 2 x 1.5 = 3 volts.  When they're about half used up they drop to about 2 x 1.2 = 2.4 volts.  Look for discharge curves on the Duracell web site.  Dead batteries are around 0.9 volts each.

 
5. White LEDs need three batteries for a higher voltage of around 3.8 volts.

 
6. NiMH and NiCad rechargeable batteries are 2 x 1.2 = 2.4 volts.

 
7. Different LEDs have different voltage requirements but they all should have a resistor to limit current.

Assembly notes:

1. Break a regular flashlight bulb, then heat it up and remove the glass and internals.  Wear safety glasses.

 
2. Bend the LED leads and fit them into the base.  Test the positive and negative connections before soldering.  My red and yellow LEDs have different configurations.  If it looks like the positive lead is too close a negative conductor, drill a hole in the base before soldering, and fill it with hot melt glue after the soldering is complete.  Mine seems okay without filling it with glue.  When it's done it looks like this.

 
3. V = IR so size the resistor to R = delta V / I

 
4. Make a resistor plate to place between the two batteries.  The resistor can be located anywhere in the circuit.

 
5. For the resistor plate, I used cardboard from a cereal box and sheet copper from a piece of roof flashing.  A coil of wire covered with solder might be easier and/or better than the small piece of copper.

 
6. I used a 22 ohm resistor for about 25 mA of current flow.

 
7. Put a 1/2 inch square piece of semi-transparent tape on the flashlight lens to make a diffuser to reduce hot spots in the beam.