Well in regard to muscle mass, I relate to skinny people, such as myself. I do benefit from more muscle mass. And just take a look at the top freedivers, do you see a skinny person?
I'm talking about becoming a body builder, just adding some strength and mass so using the big fins and swimming up from depth is easier, having some margin to sustain a reasonable good technique despite anaerobic fatigue. Having more muscles also means more blood, and when doing statics - if you relax your muscles well - that is available to you brain. These are my current views, and I love to hear arguments for and against them.
Considering all the on land things you do now, I think your benefit the most from simply trading some jogging days for swimming days.
Having a mask, snorkel, pair of closed healed scuba fins and a kickboard is already enough toys to do some serious good in water training. It's what I bring to the pool, where I only add a suit and some weights.
In the pool have a plan/theme of the day, fitting in your week/month/year plan.
If you don't know anything about your skills, test and find out and list.
Try to formulate what you like about freediving, this will give you also ideas to what a nice direction is to pursue.
Read
http://forums.deeperblue.com/beginner-freediving/90238-freediving-beginner-questions.html
Here you find much more information, also showing a link to diaphragm stretches. These are the belly under ribcage trick freedivers 'need'
to show on parties and documentaries showing their yogi special abilities to unwashed masses.
Love, Courage and Water,
Kars