Its a common myth used to sell work-out DVDs, Pilates and other similar stuff that you can "tone" muscles. Muscles do one of three things, they maintain themselves, they shrink, or they grow. When you get on a treadmill and do "cardio", your muscle doesn't go into "toning" mode and magically start to "look" better.
What makes muscles "look" better is reducing the fat around them (muscle definition), or increasing their size so they are more visible. So when doing "cardio" you are really just attempting to reduce your overall fat which subsequently makes muscle look more defined.
However for most average (not obese) men and women they will need to spend hours on the treadmill and watch their caloric intake with military precision to see much benefit. Instead if you spent that time actually building muscle they would spend less time in the gym overall, see results faster, and not have to watch what they eat as much. The more muscle you have, the more calories you burn even when your sleeping, that beats any time spent on a treadmill.
Of course virtually every women you mention this too will say: "I don't want to be muscle bound or look like a body builder, thats gross.". Its not going to happen unless you approach it like a Olympic athlete or a full time job. So you don't need to worry about this and even if you do, simply stop going to the gym and eat a few more twinkies, problem solved.
How do you build muscle while spending the least amount of time in the gym as possible? Easy, lift weights so heavy you can only do AT MOST 8 reps before your muscles give out in fatigue, then add 10% more weight to that, and don't worry about "form", thats all crap too, just make sure you don't injure yourself, which doesn't exactly set the bar very high. You want to focus on your strongest range of motion, which usually means you may only move the weight a few inches (yes, 1-2 inches is enough) just before you fully extend any joint. Heck, you could just hold the weight in one place and it'll work just as well.
Most people laugh at me when I tell them this, because since they were kids in school they were always taught "form" and full range of motion is the most important, well it only is if you want to spend hours in the gym every day for little or no results. What is most important is building muscle, and with this "idea", you should be able to increase the weight you lift for any one exercise between 5-10% EACH WORKOUT. I only go to the gym once a week now and spend less than 30 minutes. After each work-out I'm sore enough that it takes a few days minimum to recover fully so I can lift more the next time... If you can't lift more the next work out, its likely you are doing it wrong, or not waiting long enough between workouts to fully recover.
What most people don't understand is that in order for muscle to grow, you need to literally tear your muscle fibers so your body notices and starts to grow new ones. This increases the size of the muscle and subsequently the strength. Do not confuse this with PULLING your muscle, that is obviously different. But your muscles should HURT one or two days after a "proper" workout, if they don't, then you just wasted your time.
If you want to read more, google "static contraction training".