I’m using an 800 but at the weekend was running an external battery. A standard 3600mah one ran for 24 hours before I needed to change it. I had the mapping screen on a lot of the time. If you have the simple speed screens on the battery lasts a lot longer. So good if you are doing a 24h race rather than navigating a road ride you would be fine. Also turning the brightness down (on mine plugging in a power source bumps it to max!). You can also turn off the sound, Bluetooth etc. to preserve power.
Few points – I agree regarding the resetting the ride. I’d reset at intervals as well. 200miles is fine. 300 is dubious. At the weekend 2 of 5 garmins made it to 440 without crashing – 2 failed due to water damage and 1 crashed. 1000 and 810 made it. 810 and 2 800s didn’t. Also when plotting long routes this can be an issue. We had to split the ride up into three segments to stop it crashing. I believe this is a niche garmin issue as I don’t do many 440mile rides and to be honest, one is enough for me.
Be careful with the usb socket if running a battery. I think mine got water damaged at the weekend as I had the rubber flap open to plug in the charger while riding. Also if you are on rough roads/offroad there is potential to damage it that way too.
810 is less power hungry than the 1000 but not much. With both if you are on a long ride you’d need/want a power pack.
When doing more casual road rides I keep the charger in my pocket and just charge it up in the café. 30 minutes with the battery is very effective.
I have and Anker one bought from amazon.