Simulate fitness device/fitness data

I need to simulate a fitness device or fitness data to my development environment and later maybe to a hardware device.

My app is for marathon runners which want assistance to be in their optimal heart rate and find their running speed

Needed input: heartrate and steps.
For both i need to have the possibility to simulate higher heart frequency and faster/slower speed of steps so my app can suggest to not over-pacing.

Prefered by Google Fit, because i want to support and test fitness watches/tracker.
But for the beginning it would be sufficient enough to manually rise heartrate and step count and do this live so i can see my app react to this.


Thank you