Apple

Workout needs GPS signal to measure the distance so it won't work indoors and running stationary. Maybe consider an outdoors walk workout and try to walk at a rapid pace for a while to measure VO2 max - from time to time.

You won't have the GPS data

or just walk and track your walking sessions?

Thanks everyone, very helpful,

yeah, it'll want a GPS track. the primary ways it spits out a vo2max estimate is by looking at your heart-rate and pace, so it needs distance information. it also wants to know the elevation data so it knows only to spit out an estimate if the run was done on relatively flat ground (it's an estimate because the watch isn't actually capable of measuring vo2max since it can't measure your oxygen consumption).