As Alexx said, anyone can do it, it just takes hard work. And how many people do you know in this country who are malnourished!? The body is very adaptable, if you eat like a horse for 15 years it will become used to that, but then you can just as easily retrain it to graze like a cow.
Most gyms have a machine in which sends an electric pulse through your body, and measures your fat content. Or there are calipers that you can buy to measure fat. Neither are 100% accurate, but both give a good indication. I am usually about 7%.
Run to the hills! Honestly. If you do 3sessions of running a week on a fairly up and down course, you will burn the fat and get rock hard glutes. Another thing that is excellent for arse - squats. Squats build a whole lot of muscle, which then has the knock on effect of making you burn more calories all day, and squats tone arse like no-thing. :-
I personally like to do 3 sessions of weights a week, and 3 sessions of cardio, on alternate days, and this is pretty standard - unless you are a real muscle bunny like Alexx who doesn't believe in Cardio. :)
Both are excellent CV - both make you sweat, the muscle head just doesn't realise that, cos he is already wet! I personally get a whole lot more out of running, because my swimming technique is not good enough to get a really hardcore CV exercise out of it. But swimming is undoubtedly the better option for someone who is just starting out on the CV trail, as it much lower impact on the body.
Less fat? Bigger muscles? More efficient CV system? Stopped smoking? Bit warm? Could be a whole host of things!
Riding is good for burning some cals to keep the fat off, but it will never make you CV fit, or make you really stacked, because it just doesn't work you hard enough. The Gym is the only easy way to bulld the muscle you want.
Joel