Mermaid Melanie has excellent advice and Caltuna has an excellent grasp of the freediving/breathholding situation! Congratulations to you both!

I must admit that you have more dedication to this sort of thing than I, though I have at times engaged in casual breathholding to see how long I can go without having prepared or taken a good gulp of air. My latest is just over three minutes, but then my vital lung capacity (though not musculature) did partially develop like a male's.