Problem with Network Analyst

[kolombokolombo]

Hello
I create map of avaibality local bus stops. I have maps and geodatabase (street, pievements) and i make Network Analyst. Normal meters distance create the right map (screen "a"), but i have a problem with time distance. In Atribute table i recalculate shape length to walking time ["cas"] (screen "b") and take a value to the layer (screen "c"). If I give values 5;10;15 minutes program create this map (screen "d") and this is not  right for me. I need something like in screen "a". 

Thank you for a helps. 

screen a 

screen b

screen c

screen d