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• Long cardio workout sessions can cause more harm than good, leading to the release of the stress hormone cortisol, which when produced excessively may contribute to a catabolic state, in which your tissues break down, as well as chronic disease
• Too much cardio, such as marathon running, raises cardiac risk by seven-fold, and can lead to heart muscle scarring, and increased blood levels of cardiac enzymes, which are markers for heart injury
• To get the most benefits of exercise you need to push your body hard enough for a challenge while allowing adequate time for recovery and repair to take place; one of the best ways to do this is to follow a fitness regimen that includes short bursts of high-intensity activities, which can be done using an elliptical machine, recumbent bike or even free weights
• When you work out, it is wise to push as hard as you possibly can a few times a week, but you need to wisely gauge your body's tolerance to this stress, and give your body time to recuperate using rest and proper nutrition

http://fitness.mercola.com/sites/fitness/archive/2012/06/01/long-cardio-worko...