The Pebble Time 2 looks pretty tempting to develop a custom running/fitness app for. It's got heartrate detector, accelerometer, vibrator for silent feedback, plus an SDK that works with Linux.

I almost decided to make my own app recently (and was also considering the Pebble) to try and replicate the old basis Peak functionality, but when I found out that the Garmin health data was relatively open and accessible, l decided that it was easier to just make a new badge/gamification system for the Garmin data than to make an entire tracking app.

Didn't Garmin recently change the terms of their deal and put the API behind a paywall?

yes, but the GarminDB [0] project still manages to get most available data by impersonating a browser (admittedly that could be broken eventually). If that happens in the future, I'll probably revisit making an entire tracking app for myself.

[0] https://github.com/tcgoetz/GarminDB