Treat it like anything else... Set a baseline and then progress. I use Maximal Aerobic speed. Which you get by doing a 5min MAS test on something like a rower, assault bike or ski erg. for example 1000m / 300sec = 3.3ms with the aim of increasing to around 4.1-4.5ms I do off feet because of knee injuries but you can do running if you like. Then I do longer more continuous type training around 60-80% MAS for 20-30mins > move to more interval type stuff like long 3mins on 3 reps x 2 sets at 90% MAS with Rest 3min between sets. 3min active recovery between reps. > then go to more Short intervals 30 sec @110% MAS on 30 sec off x 14 x 2 sets - 3 min rest between sets. This is just how i do mine, i like having a meter target figure that i can see improve.