The GPS isn't the biggest power hog. It's the screen brightness.

You can limit the GPS power use by using an app that only gathers your position data every 3 or 5 seconds instead of constantly. For biking purposes, that's fine. It may not give you ultra accurate speed readings at any exact moment, but for mapping and course tracking and overall average speed over 100 miles it'll be fine.

And if you leave the screen off the whole time and only hit the power button when you need to check something, most phones will handle a 12 hour ride just fine.

None will last 12 hours with the screen on constantly.

If you use a dedicated bike phone that you can put into airplane mode to shut down the cell radio it'll help even more. You can also shut down all sorts of other Android services that run in the background so they don't waste battery. (No need for email and calendar syncing, etc)

But you need the right phone. My bike phone as a 3700 mah battery. It does fine. I recently tried to replace it with a small phone that has a 2000 mah battery. That was a disaster and it didn't last long at all. With the 3700 mah battery phone my GPS updates ever 1 second and I leave the screen on and it'll give me about 4 hours. If I leave the screen off it'll last me 2 days with GPS running. (Not an estimate. I've run the GPS app and left it running to see what happens. It took 2 days to run down)

If you want to use your regular phone and don't need it on the handlebars to see constantly, as long as it's in your pocket with the screen off it'll last all day.