Wondering whether to get on the elliptical trainer, or go for a hard circuit training session? The elliptical has its purposes (mostly recovery or injury rehab), but if you're looking to change your body, you've got to get the heart rate up! Oh, and keep it up. You've got to go hard to ellicit a physiological response in the body. Athletes do it every day to prepare for competition. Why shouldn't you?
Additionally, the British Journal of Sports Medicine has found another compelling reason to take it up a notch in your training. During a 16 year study of over 2500 men, they found that those who trained hard at least 30 minutes a day were less likely to contract some sort of cancer -mostly gastrointestinal and prostate cancers. The researchers found that the reduced rate of cancer was directly related to a higher rate of oxygen consumption in those that trained hard compared to those that didn't.
You don't have to go hard every day - that's probably asking too much. Three times per week is a healthy goal to shoot for, or if you only train 3 times per week, then two of those workouts should be hard.
What is hard? Try to get your heart rate up above 80% max for a minimum of 30 minutes. Intervals are usually the best vehicle for this type of training. You don't have to use the cardio machine for this either. Mix it up. Run stairs, sprints on the beach, weight or body circuits, bootcamps, etc. - there are all kinds of training routines (hey, if you're a good skier, do a mogul run for 30 minutes or more!).