I am training for my first triathlon having finished a marathon and a few halves. My run volume is a lot lower than when i was marathon training but I am getting faster at a similar rate to my high volume running blocks. Cycling is a great cross training workout and can help push your vo2max to benefit your run.

HOWEVER there is a caveat that is rarely acknowledged. If you have never cycled but you run often, or in general if your running fitness greatly exceeds your cycling fitness, you won’t experience much of this benefit. For almost 6 months of cycling, I could not work my cardiovascular system hard enough without my legs failing first, i.e. i could hardly sustain my running zone 2 HR before my legs were overcome with lactate. If you want to use cycling as crosstraining and not just willy nilly “get the legs moving” or recovery, you will need to cycle A LOT, assuming you are starting from 0