Here's why this doesn't make sense: Cardio possibilities are nearly endless.
Too often people hear "cardio" and (after they cringe, weep, gnash teeth, and maybe die a little inside) they think only of a treadmill.
Let me tell you something, if a treadmill were the only source of cardio, I would just have to kill myself. There is nothing worse than the treadmill. All you treadmill heroes, bless you, and... you can have it. "But" no. Just... no.
Cardio can be running, biking, a thousand different sprint variations (400s, gassers, fartleks, ), burpees, jumping jacks, mountain climbers, and the list literally goes on for thousands more. For that matter, almost any workout movement you do can be cardio if simply done rapidly and in chains where you don't allow your heartrate to completely recover between each
I love distance running (in the woods), but in all honestly, "just" running long distances at a near constant pace is pretty terrible (inefficient) form of cardio. Your body is so efficient that it just learns to complete to task with the least amount of effort possible and so you don't get the heart rate training or weight loss you think you're working so hard on. Then, if you couple long distance runs with a treadmill, not only do you not get the training benefits, but your quality of life just goes into the toilet as the digital distance readout sucks the happiness right out of your soul. No, really.
So, pick something else (anything else) and make cardio fun again. After all, what you're trying to do is spike your heart rate, bring it down (some), spike, fall, up, down - you're training your heart rate to rise slower and recover quicker.
I basically stood on the soap box there to remind you (or introduce you) of a fantastically efficient cardio (and full body - if done correctly) workout that actually IS a lot of fun (news flash: hating life while doing it doesn't actually make you better at cardio):
Spice up your training with the Heavy Bag. Knuckle up. Throw some blows. take out some aggression. Burn calories while you burn aggression. Have some fun.
There's nothing quite like laying into a heavy bag. Your arms get a workout, your shoulders get roasted, you rotate your hips, use your legs, tighten your abs, your lungs are screaming, your heart is taxed, and in seconds you are sweating like crazy.
Ever laugh when you heard boxers talk about how heavy their gloves feel? After a few minutes of speed work you'll swear those 8oz-16oz gloves weigh 5 pounds a piece.
Find a bag at your gym, or just buy a bag - you can get a nice heavy bag for not a lot of money. Same goes for gloves. Get a nice, padded pair of gloves ("training"/"bag"), your knuckles will thank you. I bought a new 100# heavy bag last week on sale for $65 from WalMart (the one in the picture above), then I bought a nice pair of synthetic gloves off Amazon on sale for $18 and a really nice genuine leather pair off Amazon for $40.
"What weight bag should I get?" Nothing less than 70, get the 100 if you can find it.
"What weight glove should I get?" Are you an actual fighter? If so, you don't need my advice and you're getting 16oz-20oz already. If not, I would suggest 12oz. They're heavy enough to add a little weight, to be available in nice gloves (with good padding/protection), but not too heavy to take some of the fun out of it and ruin your form. Just my 2 cents.
"I got some gloves, how do I punch?" The gloves take care of your hand form, mostly, but some things you need to think about are keeping loose until you land the punch, then after it lands, go loose again. Throw snapping punches, not pushing punches (watch this youtube video). Keep your wrist solid on the impact. Twist your punch on impact. Keep your hands up and move. Yes, even if you aren't fighting, you don't plan on ever fighting, and the bag isn't moving or hitting back, keeping your hands up and moving is a big part of the workout - maximize your efforts.
"Okay, I got a bag, I got some gloves, now what?" Start out based on your ability. If you're in terrible shape, you might start with 1 minute rounds, moderate try 2 min rounds, and I wouldn't recommend anyone doing more than 3 minute rounds. If you're good and 3 minutes is good, instead of going up in minutes, just add rounds.
"Okay... what's a round look like? Just float like a butterfly?" Try alternating 30 seconds of hard power punching, 30 seconds of speed. In the beginning you might try 15 seconds of left hand power punches, 15 seconds of right hand power punches until you get comfortable.
"I've got that down pretty well - what next?" Keep the rounds to 3 minutes, keep the 30 sec power/30 sec speed, add rounds. During the power 30 seconds, hit Hard, always combos, (touch, power, power - touch, power, power, power, etc.), and keep them coming (don't throw 1 hard punch then dance around for 10 seconds and throw another). In the speed, move up from just playing pattycake with those 7 pound hands to actually making the speed punches pop. In both speed and power, swap striking location often (head, body, ear, groin, kidney, etc).
It will be a TON of fun... AND crushing. You'll love it.