QUOTE (GreekWarrior @ Oct 7 2009, 07:06 PM)

^^^ exersices that use more than one muscle group

Thats a real squat, all the way down.
As for fitness HIIT, fartlek or interval training is where it's at. Your heart rate needs to be up to 80% of your max heart rate (for easy calculation max HR is 220 - your age. then your target fitness zone is 80% of that)
Boxing, kick boxing e.t.c is excellent for fitness. You go all out for 2 to 3 minutes then take a 30 second or so rest, then your at it again. You can't keep your heart rate at 80% for long periods of time so it's ideal.
Fitness is basically a measurement of your bodies efficiency, long distance bikes and runs will build endurance (sort of a derivative of fitness, but you notice your heart rate probably hovers between 140 to 160 or so Bpm, 60% to 80% HR) but the previous given methods will make your body more efficient. It will also stop you from stalling, you will find it hard to break a plateau with constant pace workouts.
Unless you are training for a specific long distance event, there are better ways to boost your fitness.
Edit: if you want to keep a constant pace workout, slam some hills into your rides and runs if you can, go full bore to the top then cruise down, if you want to do it that way.
Edit again: Don't use the smith machine to learn to squat (They don't follow the natural curve of the squat movement and place excess stress on the knees... in fact for best results stay away from machines altogether) Start off in a rack with an empty bar. In the beginning it will feel very awkward and you will make technical errors. If you can, have someone watch you do them and they can correct you. You will probably find you can't go down very low at first, but you will get used to it. You want to get to the point where you can go as low as you can (comfortably, everyones different), with correct technique before throwing some serious weight on there.
Things like dead lifts, sit-ups and Hyper-extensions will also help strengthen your core muscles, which will prevent back injury.