GPS for all day rides

I think the 510 (520?) is the

I think the 510 (520?) is the best of Garmin’s bunch here, but 20 hours+ might be a struggle from most makers. 

From memory, when I looked, mio and lezyne also claimed around 20hrs, but Bryton was the standout with 30-odd hours (I have never seen a Bryton unit, nor known anyone who’s had one, so no idea what they’re like). I would imagine connecting to sensors and using the backlight would reduce that quite a bit.

Are you going to want to connect to HRM, cadence/speed sensors? If you don’t need all that stuff, a spare battery or a cell-powered charger for your phone might be the way to go – and probably cheaper.