Zombies Run, Help they get me!

Hi runner fives. Can anyone tell me why I can no longer out run the zombies? With effort, I used to be able too but I think one of the recent updates must have sped them up as they always catch me now.
I run on the spot and use the step counter with the zombies chases enabled. I must also admit that I have never understood the stride length and if that may affect the chases?
Any help welcome.
Thanks
Mark