Within London, all London Underground, National Rail, London Overground, TfL Rail and Docklands Light Railway stations are assigned to six fare zones. Fare zone 1 covers the central area and fare zones 2, 3, 4, 5 and 6 form concentric rings around it.
The theory is that you use Z2 to get in the training without overtaxing yourself and therefore give your body the proper recovery it needs to make your hard days hard. Z3 used to be referred to as “junk miles” because they were not hard enough to stimulate growth/adaptation, but not easy enough to facilitate recovery.
There are ways to estimate HR zones including the formula 220-age to find out maximum heart rate. Zone 2 is approximately 60-70% of maximum heart rate. For me, HRmax would be 220-30= 190. However, this calculation can be off from true HRmax by 20 bpm.