navigation in fenix 6 / tactix

Hi, 

could you tell me how navigation in Tactix / Fenix 6 works... I noticed that when I click on navigation and chose city located about 100 km from my current destination, watch is calculating route with endless time :( So my question is: how to use navigation in Tactix / Fenix 6? What is a reason for option navigate to the city if watch is not able to calculate a route? 

Thanks