I see Viewranger can download 3 routes and offline maps onto an Apple Watch – anyone doing this that can report on battery life?

My iPhone using Outdoors GB won’t last for more than 7 hours, not enough for the South Downs Way! I’m wondering whether Viewranger on an Apple Watch would last.

The alternative is a Garmin 820 I guess?

Not got one, but the reports I’ve seen suggest that you wont get much more than 5 hours running GPS on the Series 2 Apple watch. If the app were smart enough it might be able to hand off to the phone when the watch got low and vice-versa, to extend the overall time, but I’m not sure if any of the apps are smart enough to do that yet.