Usually when I get to the health club or outside to jump on my bike I'm mentally ready to go and can't wait to start, and that's compounded that with being perpetually short on time.
However there is something you really should do first, and that's warm up.
Warming up makes it less likely you'll injure yourself, and although many people know that, they often warm up wrong.
Stretching doesn't constitute warming up, although that's a popular misconception.
Warming up is a few minutes, around 5 to 15, of light aerobic exercise.
The less fit you are the longer a warm up you'll need according to exercise experts.
If I'm at the gym, I may take a few slow laps on the track or walk gently on the treadmill.
If I'm biking, I'll pedal a mile or more gently and slowly on flat terrain.
If I'm running, a common warm up is to walk a few minutes.
After your workout you should cool down.
Cooling down is simply a few minutes of easing out of your workout.
Not cooling down can have negative consequences, even placing undue stress on your heart if you're really out of shape.
If I'm biking my cooldown might be a couple of miles of very gentle peddling.
If running, perhaps a slowdown culminating in a walk for a few minutes.
You really should be warming up before and cooling down after your cardio workouts.
Skip them at your own risk, and remember that if you don't have time to warmup and cooldown you really don't have time to workout, at least not safely.