My Bangle JS2 battery has gone 37 days and is at 9% now

As I understand it the Run+ app is basically the Run app with an extra screen, and the Run app uses Layout which should update lazily. In fact there was an issue a few weeks ago where the Lazy draw was meaning the screen didn't update correctly after a prompt was shown so I think we're good.

But exactly as @HughB says:

The LCD backlight is the biggest power draw in the system so if it comes on by accident when you run, that will really hurt battery life.

And GPS is second largest power draw - I guess it might be possible to implement something in the exstats module as @HughB says (to ensure power usage is lowered) but potentially that will hurt the metrics you see on-screen like speed.

You should also make sure you use the AGPS app and set up just GPS as the source, as that can save quite a lot of GPS power consumption.

It should be possible to configure the GPS itself into a low power state, but the documentation for the chip really isn't great and unfortunately I haven't been able to find out anything useful