[auxym]

According to this [1], running is much more efficient at improving cardiovascular endurance than biking, in terms of time spent training for a particular gain. Particularly trail running.

[1] https://www.amazon.com/Training-New-Alpinism-Climber-Athlete...

(ps. As someone who is sort of a weekend outdoors entusiast, without being any kind of a competitive climber or particularly interested in training, I thought it was an interesting read and would recommend.

[rdmsr]

It's a solid book, although I don't recall the bit that you're citing. I didn't read it that thoroughly. Started it and realized that I'd rather work towards 5.13 than the summit of Everest/Denali/whatever.