Mine keeps on going as long as I put new batteries in it. Do you not take a break at least once every 20 hours or so?

If you really want to do something like 20+ hours without changing batteries and with enough brightness to see around on easy routes then I'd probably be looking at lights that utilize li-ion cells or packs.

I have a single cell 18650 powered light that I use as a bike light, camping light, running light, headlamp (for your head) etc. Turning it down to 150 lumens would be 11 hrs, good enough for any single trip for me. If you want to turn it down further, the 65 lumen setting is good for 30 hrs, but you start getting questionable brightness to be seen. I have 4 cells though, so I don't worry about battery life.