LED 10mm 8400mcd loses brightness using battery A23 12V
THIS A23 battery?

To get 48 hours out of it, according to the datasheet there, you'd better only be taking about 0.5 mA from it, which is 2.5% of the rated current of the LED.

I don't think it's the LED you need to worry about, I think you need to change the battery. (although for road use, that seems like a very low power LED too) and 2 Megohms doesn't sound right either, whatever circuit you used. Where did you get this design from?