@Joakim_Lind
Zwift can control resistance on smart bike trainers, but it took years for the bicycle industry to get to where it is now. Happy 7th birthday to Zwift this month, incidentally!
On the running side of the game - Zwift can not control a treadmill’s speed or incline. The treadmill industry’s integration with third-party apps like Zwift is still in its infancy. So this might happen in the future, but it’s not available today.
Maybe you need to have a look at Runcline or qDomyos-Zwift, both these apps send speed from the treadmill to Zwift and that works without problem, but Zwift is not sending the speed data for example
in a workout back to these applications, like it does when you ride a bike. So my suggestion is that Zwift should start to send speed/pace back to the treadmill/application