How can you know if you are working in the right intensity? Using an RPE (Rate of Perceived Exertion) chart can help you to determine the appropriate intensity. The scale uses a 1 to 10 rating system. One is very light, such as walking to the refrigerator for a glass of milk. Ten would be a very significant level, representing maximal exercise. Ten would be indicative of not being able to take another step without fear of collapse. It is not recommended for anyone to work at a rate of 10 without strict supervision by a healthcare provider. Moderate intensity is the level of exercise that is most recommended, and can be determined by a rating between a 3 and a 5.

Only one day – as is not entirely unusual – my bus simply did not come. I waited and I waited, and I waited some more, my blood pressure rising, spitting and swearing and huffing and puffing over the unimaginable injustice of a BUS THAT WOULD NOT COME, alongside gathering hoards of similarly frustrated non-passengers… Then, after 20 minutes, spurred onwards by a desire to demonstrate I simply would not stand for such abysmal service! - I walked.
All that oxygen being pumped by the blood is important. You may be familiar with the term "oxygen consumption." In science, it's labeled VO2, or volume of oxygen consumed. It's the amount of oxygen the muscles extract, or consume from the blood, and it's expressed as ml/kg/minute (milliliters per kilogram of body weight). Muscles are like engines that run on fuel (just like an automobile that runs on fuel); only our muscles use fat and carbohydrates instead of gasoline. Oxygen is a key player because, once inside the muscle, it's used to burn fat and carbohydrate for fuel to keep our engines running. The more efficient our muscles are at consuming oxygen, the more fuel we can burn, the more fit we are, and the longer we can exercise.
Whatever makes you feel comfortable is the easy answer. There is no need for fancy spandex or workout clothes, unless that’s what you like. As you start moving farther and faster, you may want to get dedicated fitness walking clothes that wick away sweat or allow you to layer for different weather conditions, but for a beginning fitness walker, comfort trumps everything else.
Aerobic exercise (also known as cardio) is physical exercise of low to high intensity that depends primarily on the aerobic energy-generating process.[1] "Aerobic" means "relating to, involving, or requiring free oxygen",[2] and refers to the use of oxygen to adequately meet energy demands during exercise via aerobic metabolism.[3] Generally, light-to-moderate intensity activities that are sufficiently supported by aerobic metabolism can be performed for extended periods of time.[1] What is generally called aerobic exercise might be better termed "solely aerobic", because it is designed to be low-intensity enough so that all carbohydrates are aerobically turned into energy.
Utilize the Incline Feature: “Treadmills have a variety of uses and one of them that many people don't take advantage of is the incline feature. Walking or running on a treadmill is meant to simulate how you walk or run outside. The incline feature turns what could be compared to walking or running on a sidewalk, into a motion more similar to walking up a mountain,” says Crockett. “Adjusting the incline on a treadmill has several benefits. Your workout intensifies because you're placing more demand on the body to keep up with the machine. The higher you set the incline, the more energy your body is forced to use to help activate your glutes, quadriceps and calves, all of which have to put in overtime when the incline is increased. This increase of energy burns more calories and depending on your weight and cardiovascular endurance can also burn more fat.”

For weight loss, gradually work up to 45 minutes or longer at moderate to vigorous intensity five to six days a week, allowing for at least one day of rest a week. Vigorous intensity refers to an activity that will have your heart beating quite a bit more than moderate intensity workouts, and your breathing will be harder so saying more than a few words will be difficult.
One program created by Dr. Hiroshi Nose and colleagues at the Shinshu University Graduate School of Medicine in Matsumoto, Japan consists of repeated intervals of three minutes of fast walking, aiming for an exertion level of about six or seven on a scale of one to 10, followed by three minutes of slow strolling. As reported by The New York Times:13

When you slap your foot on the ground like some undifferentiated plate of meat, it negates all your ability to use the muscles at the back of your body, and loads all the work on to your hip flexors. Feet have the same structure as hands – tarsals, metatarsals, phalanges. We should experience the same articulation, the same wriggling and precision, the same fine motor-skilled attention to the detail of the exterior world, but we don’t: it’s more of a slap, slap, slap, or at best, a pad, pad, pad.