Porky,
You are correct - first, BE SAFE. dive with a buddy who knows about SWB and how to treat it.
Spend some time perusing this very training forum. You will pick up a lot in these threads even after sifting though all of our BS.
Also check out <
http://www.freedivehawaii.com/training/matt_briseno.asp> for training ideas.
Joining an underwater hockey club is a good way to improve your snorkeling skills and build up some CO2 tolerance.
First, I think that being a competitive swimmer, you want to concentrate most of your efforts on learning breath control and relaxation. You are probably already in sufficient cardiovascular condition that you will see the most improvement by learning to slow your heart rate and relax your overall muscle tone in order to reduce oxygen consumption. Try Yoga. The little I've done convinces me it is the most productive use of training time. Remember that unlike competitive swimming, freediving is not about exertion . . . it is about minimizing exertion and maximizing concentration. I think you will find that many others on this forum agree with this priority . . . and although I am not one of them, there are some AMAZING freedivers who participate in this forum.
I'm also a former competitive swimmer and would suggest that you continue your aerobic training, but incorporate more apnea into the sessions. For example, at the end of a swimming workout, I typically do a set of 50-yd. kicks with fins (training fins, not long blades) on 1:00 or 1:10 with the first 25 underwater and the second 25 on my back recovering. Then I might do some 25-yd fin sprints underwater concentrating on the streamline and on powerful kicking, resting 25 seconds or so between each repeat (mostly to help my UWH). Finally, I might do some slow 50 - 75 yard fin kicks underwater trying to make the entire 75 yards on one breath. I rarely make it, but when I have to surface, I allow myself only one breath and then finish the 75.
I also incorporate 3-4 minute bouts of breath-hold intervals when I jog. I do 15-20 seconds breath-hold each with an equal amount of free-breathing between. So while jogging, I hold for 15 seconds, breathe for 15, hold for 15, breathe for 15, etc. for a period of 4 minutes; walk for 1 minute, then I jog normally for another 4 minutes, then repeat. Don't overdo the breath-hold . . . you don't want to get dizzy and kiss the pavement . . . tends to mess up the way your mask fits!
hope this helps. good luck, and dive safely.