I've been thinking of treating myself and replacing my Polar A300 activity tracker watch (when finance allows), so have been looking at fancier alternatives. I didn't spend long looking at Apple as they have a bad reputation for battery life (and cost the earth) - they claim 18 hours mixed use for the Apple Watch 4, but only 6 hours of "continuous outdoor activity tracking". I found
an article where someone tested one on a hike. They did 6 hours 20 minutes of hiking and the battery dropped to 5%.
By way of comparison, Polar claim that their Vantage M multisport watch with GPS lasts up to 30 hours in training mode (GPS and wrist-based heart rate).
PS - the Polar watch gets some pretty bad reviews on Amazon though!
Edit - after reading reviews and looking at prices, have decided to stick with my Polar A300. No GPS, but works well as an activity tracker and watch, and the battery lasts WEEKS.