I like a feature when watch detects that one is walking and offers to record the walk from very beginning... but unfortunately this is not always working and there is no way to initiate this manually. Hopefully they will fix this with the next release.
Open the fitness app and start a workout - outdoor walk. The first time may be a PITA, but after that it will be in your cache and on the main menu. I know because I walk every single day and in two taps I can get my workout started and recorded.