Working on cardio is probably not going to help with air consumption. Possibly even the opposite! Achieving good air consumption is largely a matter of technique, though physiology has a small part to play.
The first and most important thing is to get your buoyancy right. If when you're swimming along a wall, if you stop swimming do you start to sink? That's what many people do, and that's because they're used to being overweighted and swimming upwards at the same time that they're swimming forwards. Get someone to video you and you should be able to see it for yourself. Get used to what being horizontal feels like, and adjust your buoyancy until swimming or not makes no difference to your depth. If you find you're using your hands or arms to scull for position at any point during a dive, you're not weighted correctly.
Secondly, and closely linked with the first, make sure you're not carrying excessive lead. If (after adjusting your technique as above) you always have some air in your BC then you're probably carrying too much lead. Reduce it, a lb at a time, until you reach the point that maintaining a stable 15ft at a safety stop with your tank at 500psi is a bit of a struggle. Don't dump any more weight, but experiment with your breathing to improve your stability at the safety stop. Only when you've exhausted your breath control options and you're still struggling to stay down add back a single lb of lead. Obviously if you change your equipment, especially your exposure suit, you'll have to repeat this process, but after a while you'll find you can do it very quickly.
Make sure your finning style is effective and that you are wearing suitable fins. For your build I'd go for one of the stiffer fins such as the Mares Quattro (and the two and three-slot versions of that fin). Avoid soft floppy fins and IMO steer well clear of split fins. You need to fin from the hips and ankles, not the knees, and you should feel resistance to the blade of the fin as you push it. If you don't feel resistance your finning style isn't right - you may well be "cycling", which is a jolly good way to use up all your energy and air without actually going anywhere! Again, get someone to video you. Get used to the feeling of slow but measured forceful strokes with coasting breaks between them.
Make sure your breathing is deep and slow. If it isn't and you don't know how to change it, concentrate on making it deep. The slowness will come automatically, but you'll need to work at it to perfect it. Don't actually skip-breathe, but at the same time don't exhale deeply as soon as you've just inhaled. Let it out slowly, over quite a few seconds. Also, when you're working hard don't breathe in, and when you're breathing in don't work hard - the combination makes you take in far more air than you need, which you then exhale without getting much benefit from it.
Lastly, try to make sure gear and weight is distributed around your body so as to help you remain horizontal. Don't have all the weight low down on your hips and all the air high up by your shoulders - that'll push you into the "begging" position which might be great for looking at something on a wall, but is desperately inefficient for swimming along it.
As a guide to air consumption and a target to aim at, I'm 6 inches shorter than you and a little bit lighter. For the standard BH dive I use an 80 cu.ft. tank and usually surface with about 1800-2000psi left. But I do have one or two more dives than you.
The first and most important thing is to get your buoyancy right. If when you're swimming along a wall, if you stop swimming do you start to sink? That's what many people do, and that's because they're used to being overweighted and swimming upwards at the same time that they're swimming forwards. Get someone to video you and you should be able to see it for yourself. Get used to what being horizontal feels like, and adjust your buoyancy until swimming or not makes no difference to your depth. If you find you're using your hands or arms to scull for position at any point during a dive, you're not weighted correctly.
Secondly, and closely linked with the first, make sure you're not carrying excessive lead. If (after adjusting your technique as above) you always have some air in your BC then you're probably carrying too much lead. Reduce it, a lb at a time, until you reach the point that maintaining a stable 15ft at a safety stop with your tank at 500psi is a bit of a struggle. Don't dump any more weight, but experiment with your breathing to improve your stability at the safety stop. Only when you've exhausted your breath control options and you're still struggling to stay down add back a single lb of lead. Obviously if you change your equipment, especially your exposure suit, you'll have to repeat this process, but after a while you'll find you can do it very quickly.
Make sure your finning style is effective and that you are wearing suitable fins. For your build I'd go for one of the stiffer fins such as the Mares Quattro (and the two and three-slot versions of that fin). Avoid soft floppy fins and IMO steer well clear of split fins. You need to fin from the hips and ankles, not the knees, and you should feel resistance to the blade of the fin as you push it. If you don't feel resistance your finning style isn't right - you may well be "cycling", which is a jolly good way to use up all your energy and air without actually going anywhere! Again, get someone to video you. Get used to the feeling of slow but measured forceful strokes with coasting breaks between them.
Make sure your breathing is deep and slow. If it isn't and you don't know how to change it, concentrate on making it deep. The slowness will come automatically, but you'll need to work at it to perfect it. Don't actually skip-breathe, but at the same time don't exhale deeply as soon as you've just inhaled. Let it out slowly, over quite a few seconds. Also, when you're working hard don't breathe in, and when you're breathing in don't work hard - the combination makes you take in far more air than you need, which you then exhale without getting much benefit from it.
Lastly, try to make sure gear and weight is distributed around your body so as to help you remain horizontal. Don't have all the weight low down on your hips and all the air high up by your shoulders - that'll push you into the "begging" position which might be great for looking at something on a wall, but is desperately inefficient for swimming along it.
As a guide to air consumption and a target to aim at, I'm 6 inches shorter than you and a little bit lighter. For the standard BH dive I use an 80 cu.ft. tank and usually surface with about 1800-2000psi left. But I do have one or two more dives than you.
