I want to try it but how am I supposed to run at the same speed as I walk?

I mean physically, what am I supposed to do to run at 5-6km/h? Should I exaggerate and slow down my run stance? Like I'm fake running? Is that more demanding than walking and a good intro to running vs just running/jogging?

This is called "jogging" not "running" and it is perfectly fine to do it a pace less than a medium-brisk walk.

I wouldn't get too hung up on actual speed or numbers, but try to just keep it slow.

I hear what you are saying. Running gait is more sort of a lean-forward-and-push-off-with-toes type thing than a smooth easy walk. You can do that with short strides and medium-cadence and you won't go very fast.

Perhaps just transition into a run and then make a conscious effort to slow down, or go the other way where you walk faster and faster until your instinct is to change to a running gait and try to keep that pace.

The important thing is to try not run too fast "by accident".

Good luck