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.