Dude!!! Paise zyada ho gaye ho to mujhe de do.
You don't need a treadmill or a gym for getting exercise, no matter what your fitness goal is (unless you have some high-level performance requirements like competitive sportsmen competing at the national or international level, which obviously you don't).
Some background information first.
W.r.t. exercise, there are 3 main objectives: strength, cardiovascular performance, flexibility. If you want a lot of any one, you will need to develop at least a minimum level of the other two as well. Since your question is about treadmills and running, I will assume that your focus is cardiovascular performance (i.e. increased heart and lung capacity, and blood's oxygen carrying capacity). The strength requirement means that you will need to make your legs stronger by specific leg exercises (squats, leg raises, etc.), and flexibility requirement menas that you should tretch adequately befoere and preferably after the cardio exercise.
For improved cardiovascular performance, the exercises people usually use are: running (outdoors, on treadmills), walking (brisk walking, power walking), swimming, cycling (on an actual cycle outdoors, or on a cycling machine in a gym --- nowadays in the US it's called "Spin classes" I think), rowing (in an actual boat in a water body, or on a rowing machine in a gym), climbing stairs up & down (physically in a building, or on a machine called a Stairmaster [one brand I suppose]). And there is the jump rope i.e. skipping, that someone else mentioned. For each of these, you can do them at different levels of intensity; so some old man or fat young woman may walk 1 km in 20 minutes and a young football player may run 1 mile (= 1.6 km = 4 laps) in 6 minutes, and both may be praised for that. Because any exercise is supposed to be done upto just beyond the person's current capacity whatever that may be; only then will the exercise deliver benefits. But all other things being equal, different exercises are good for different things, they each have their advantages and disadvantages. Running, walking, stair climbing and cycling will strengthen the lower body (legs , lower back, abdomen), whereas rowing will develop arms chest and upper body more than legs, and swimming will strengthen both but not equally. Cycling has the highest calorie burn rate per minute, butthat is on a racetrack or on a cycling machine in a gym, not on a cycle outdoors because there you are not going at your fastest speed possible due to being constrained by the environment (traffic, road conditions, obstacles, terrain, view, etc.). Since you can go at your peak speed, running and swimming have the highest practically achieved calorie burn rate per minute, and hence are best for removing fat (i.e. weight reduction). But running does this job better than swimming; that is the anecdotal evidwence at least. Cycling doesn't help reduce fat on the belly even though it removes fat everywhere else. And another aspect to look at is which body parts get stressed by which exercise. Running and skipping (jump rope) involve impact so load on joints especially ankles and knees is very high (6 times body weight on each knee in case of running). Doing squats (baithak) stresses the knees very much, and even more so if you do weighted squats i.e doing squats while holding dumbbells in each hand over shoulders, or holding a barbell across your shoulder behind your neck. Climbing stairs, or even getting down stairs, loads the knees, because when you lift one leg to put it on the next stair (up or down) your whole body weight is on one leg and that leg's knee is bent and moving while taking the whole body's weight; the faster you do this the more the load on the knee --- that is why it is easier to climb up 2 or 3 stairs at a time than to climb up one stair at a time to achieve the same speed.
My point in telling you all the above is, in selecting the exercise to do, you need to look at what specific body weaknesses you already have or are likely to develop (due to heredity , say) and which exercise causes least injury. At the very least, you want to avoid impact; that would eliminate running and jump rope. Swimming and rowing require either a gym or a boat + lake/river, so both are expensive and difficult if not impossible. Walking is good IFFFFFFFF your fitness level is so low that your heart and lungs and blood are stretched to their limits by walking itself. But if you find that you can jog comfortably without getting exhausted, then walking will not do you much good as an exercise. Climbing stairs and cycling you can do in your daily life, they are cheap and easily accessible, and will get you a minimum level of physical activity even if you don't get any exercise separately deliberately. So start with taking the stairs everywhere, and in your office or home building if there are say 4 floors, then climb up and down between ground floor and 4th floor 5 times without stopping as fast as you can. And buy a bicycle (cost Rs.4500 for a decently good 20-inch one) and ride it to and from work everyday if traffic and distance permit (8 km can be covered in ~25 minutes), and if that is not possible then ride it for 30 minutes continuously everyday (15 minutes to go 5km away, and 15 minutes to come back).
Now to your actual question. For your cardio vascular exercise, do Suryanamaskar.
For a starting point, look at this.
The described Suryanamaskar A is simpler than Suryanamaskar B. If necessary you can make Suryanamaskar A even simpler (which I did since my knees hurt and I have to avoid load on the bent knee and all jumping/impact). Initially you start with a small number of repetitions (say 5 or 10 or 20 Suryanamaskar, whatever is your capacity), and do it slowly BUT CORRECTLY i.e. exact positions and shapes and sequence. Once you are able to do that many easily, you can increase the number, say by 1 every day or 10 (or 20 or 40) every week. How do you know how much to increase by? The answer is, to always do *just slightly* more than what you can do comfortably; and in general don't do this on consecutive days. That means you should do this maximum 4 days a week. As you go along, you can increase the speed too. All instruction for Suryanamaskar tells you go slow, hold every pose for a few seconds or breaths and feel eevry muscle and body part etc. and all that is useful and good and has its own benefits. But if your target is to do Suryanamaskar as a substitute for running, then you need to go fast enough to stress out your heart and lungs. For me that meant staying in each pose for 1 second, and finishing one full cycle in 18 seconds; any traditional teacher will say that this is too fast and actually not Suryanamaskar. But undeniably, what it is is pushups and raising your body centre of gravity from floor to standing and back; it is resistance training or weight training done fast enough as a cardiovascular exercise. And I can say from experience that it is better than running and cycling. I have done more than 180 Suryanamaskar (at 18 seconds each, total ~55 minutes) and at the end my heart rate achieved (160-165 beats/minute I think) was higher than what I achieved by running or cycling. So definitely for performance and benefits, Suryanamaskar is a good alternative to running (and walking, cycling, swimming, rowing, skipping, climbing stairs).
Advantages and other points of importance.
1] It doesn't need much space. The floor space needed is as much as your height with arms outstretched; the vertical space needed is also the same. So if your height is 6 feet, then a 8x3x8 feet (length x width x height) empty space in your house (in a room with a window and a fan) is all you need.
2] It doesn't need any equipment. You don't even need a yoga mat. If your floor is stone or ceramic tiles or even better some rough surface like shahabad tiles then you can do the exercise directly on the floor.
3] It doesn't need any special clothes. Actually it is best done naked. I am not joking. Doing Suryanamaskar naked, with one window open to the outdoors and with the fan on AT FULL SPEED, solves 2 problems for you. Firstly you will drip less sweat because your body gets better cooling by sweat evaporating directly without collecting in liquid form, and so your hands and and feet have less sweat on them so you get a better grip on the floor and can do the exercise better. Secondly the sweat doesn't get absorbed into clothes and so you don't have to wash the clothes immediately so that they are ready for use the next time. That said, even naked, you will sweat A LOT. I had to wear sweatbands on my forehead and both wrists so that my palms didn't slip on the floor and sweat didnt get into my eyes. And after every set of 20-25 Suryanamaskar, I had to take a 1 minute break, not because I was tired, but because I had to wipe off the sweat on my body. By the time I had finished all my exercise for the day, a towel would be soaked wet with sweat. And when I left that towel in water to wash, the water would become opaque (lightly milky) as the sweat from the towel mixed into the water.
4] It can be customised to accomodate any injuries or weaknesses you may have. My issue was both knees and one ankle, so I wanted zero impact and no load on bent knees. Someone else may have a bad back or fractured wrist from the past. These things can be worked around. The objective is to reach and stay at the limit of the heart and lungs for as long as needed, and there are multiple ways to achieve that.
So in sum, don't buy a treadmill, don't run. Buy a bicycle and ride it everywhere you can, climb stairs everywhere you can, and do Suryanamaskar at home.