Breathing is basic to harp playing, and rhythmic breathing should be part of every practice session. Start by using rhythmic breathing patterns to play draw and blow chords on holes 1-3. Practice as fast as you can, but not faster than you can. If you find yourself stumbling over the pattern, slow down and work it into your muscle memory. Be sure to relax, open up your throat, and use your diaphragm to control your breathing, not your lungs, mouth, or tongue. This will improve your resonance and the depth and richness of your tone.
Fast improvisation can be achieved by using repeated breathing patterns to select the notes you play, rather than selecting notes some other way then doing whatever you need with your breath to get those notes. Of course this style should not replace other improvisation, or dominate, necessarily, but can add to other styles and put another tool in your bag of tricks. Seems to me that John Popper uses this approach routinely.
The other point with respect to breathing patterns is that analyzing difficult passages to identify the underlying breathing pattern can greatly simplify the learning and playing of the passage.
There are superimposed rhythmic patterns at work when we play: