Coding an app for running and cycling ?

This looks great! There is an app for Bangle.js 1 called 'BangleRun' which provided some useful functionality like this, but ended up being a bit bloated for what it was (duplicating all the GPS recorder/interface functionality). Making a version for Bangle.js 2 (and backwards compatible with 1) is high up my priority list and I have a bit of a prototype here.

... but it didn't provide exactly the functionality that's being suggested here, so the more the merrier really.

For GPS recording, please just use the Recorder app though - once it's installed, your app can just do:

if (WIDGETS["recorder"])
  WIDGETS["recorder"].setRecording(v).then(function() {
     print("Recording!");
  });

For heart rate zones, it might be worth adding those calculations based on the recorded track, as a separate app or as an additional way of viewing data from recorder. That way it'll work with any tracks that get recorded.