I read the book and started trying out the methods whilst recovering from an operation, which probably wasn’t the best start, but seemed like a good idea at the time.
I couldn’t maintain the cold showers. Not even on my arms and legs, not the operation bit 
I still believe cold exposure could have positive effects, so my plan is to do more open water swims from the spring. I have entered my first short distance, off rd triathlon in July, so it gives a purpose and hopefully motivation for cold exposure.
I found the Wim hof breath holding method to have an immediate and quite profound mental benefit. However, this was followed by a headache each time, so I decided to shelve it until fully recovered.
Coming back to it, I started reading James Wilson’s take on it:
What I like about it, is that it can be integrated into what I’m already doing - walking building to running. (Sometimes I feel I do so many aspects of training/recovery that the length of the routines become kind of oppressive, so it’s good to have something that integrates with what I’m already doing).
When I do the James Wilson approach, (even just walking) the headache is immediate. I’m going to try a short breath hold workout mid walk and see if I can build up from there.
I used to get a headache from sudden intense cycling - like a 10 mile TT/1 hour mtb race. What helped most with this was V02 max training. From what James Wilson has to say on the subject, my headache would suggest that my body is currently not making the most efficient use of the oxygen I’m taking in.
As I regain my fitness, I’m likely to get better at breath holding. Will breath holding work help me regain my fitness…? I like to think so.