Ah, thanks so much @ddmiller for the tip - configuring the Action Button makes starting the workout so much easier! That would have saved me so much fiddling and frustration, but of course will be beneficial going forward 
Hear hear, and in fact I have been thinking about the thread you started. The G-Shock 9400 Rangeman and many other watches have the facility to calibrate the altitude and barometric pressure, so it is very unlikely that this capability is not possible in the Apple Watch.