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@MikeTheBear I gained my understanding on the topic of the mysterious realm of cardiovascular training from the "Cardio Code". Great resource which takes physiology into account and a high number of scientific studies on the subject. With his sophistication Kenneth Jay wants to build up VO2 max mainly via expansion of the heart walls to increase cardiac output. His protocols are based on all out testing for instance 2000m row, Wingate test on a bike, 12min Cooper running test to know the percentages one has to use for interval training protocols with built in retesting. To me it sounds interesting for a serious endurance athlete, for me as a recovery and relaxation runner I have no real business with it.
My take home point simply is, that if I want to to have relaxation and expansion for mind and heart, building capillaries, the mysterious mitochondria I need to row some or getting on a run. As my paradigm I choose MAF or lower, as I found it helpful for recovery and sustainable over the last two years and I am glad that Al proposed to do so then.
When I started Kettllebell training 9 years ago I stopped cycling and running abruptly. Currently I am fond of having a simple MAF run of around 45min quite frequently. It pairs well with the bells and basic strength training.