I haven't started a disciplined regiment of breath work yet but I have been on deathly-cold showers for a year or so and expose myself to uncomfortable cold whenever possible!
still going...at this stage I really do think the process has saved my life.
I doubt that I would have made it through the last 10 weeks alive and employable if it weren't for doing this (plus a whole bunch of other meditiations.
This is this morning (breath hold, not on exhale, but after a couple of two minute holds on the exhale, using this routine as the baseline (three runds), then using the starting breathwork before in inhale hold.