I'm intrigued by this question because I consider "flow" to be more associated with mentally demanding tasks than physical activities. I most commonly experience flow when coding. It's a state of almost anti-distraction, because the only thing I want to do is just the task at hand, in the most extreme way. Even the need to sleep becomes an annoyance! The main thing is that the most fun, interesting, stimulating thing you want to do is the task at hand, you're totally absorbed in it, and it's really unpleasant to be removed from it.
That said I do recognise a slightly different concept from what you're saying about physical activities. There have been times where I've done longer periods of exercise of moderate intensity exercise (e.g. 45 mins on a run) and experienced that state of your mind being 'totally blank'. But I would say the experience is a bit similar to mindfulness breathing exercises, where focussing in one thing your body is doing - maintaining your cadence, or just focussing on your breaths - achieves an inner calmness, almost an absence of thought.
By contrast, when I'm in mental "flow" actually my mind is full of thoughts. I'm confronted with an interesting problem and I just have to solve it. If I stop working on it my mind is just spinning with stuff related to the task at hand.