I've been testing Apple Watch 3 with cellular for battery life over runs and cycles over the last week. I'll be reporting back on that once I have get a few more hours under my belt and tried various combo's of BLE HR strap, Music play over AirPods, Low Power Mode, Cellular Data on/off etc...
However during my 4h 30m cycle on Sunday I wanted to charge the watch a little at the coffee stop. It was sort of awkward using the standard Apple Watch cable (even with the 1m one), with a portable battery bank next to my coffee and beans on toast!
So I went looking for a different solution and came across this one on Amazon
When it arrived today it was a little larger than expected, however it came with nice unexpected benefit if you have one of the new 42mm Apple Sport Loops - you can stretch it around the battery and it is a nice fit and will stay put.
It should fit in the back pocket of a cycling jersey for longer rides until I find a better solution. For running a marathon - not so sure as yet :)