got this from wiki. its an article on aerobic exercises.. link:
Aerobic exercise - Wikipedia, the free encyclopedia
here are the criticisms of running and such but there's nothing about lowering testosterone levels and i don't see how it would to be honest...
and i suggest changing up your routine... throw some interval sprints in there maybe 90 seconds sprint, walk 60. do this for 15 to 20 minutes and throw in some long, easy runs every now and then too... cardio will get old really quick if you don't switch it up imo.
Criticisms
When generalized fitness is a professional operational requirement, as for athletes, combat services, police and fire personnel, aerobic exercise alone may not provide a well-balanced exercise program. In particular, muscular strength, especially upper-body muscular strength, is usually neglected. Also, the metabolic pathways involved in anaerobic metabolism (glycolysis and lactic acid fermentation) that generate energy during high intensity, low duration tasks such as sprinting, are not exercised at peak rates. Aerobic exercise is, however, an extremely valuable component of a balanced exercise program and is good for cardiovascular health.[citation needed]
Some persons suffer repetitive stress injuries with some forms of aerobics and then must choose less injurious "low-impact" forms or lengthen the gap between bouts of aerobic exercise to allow for greater recovery.
Aerobics notably does not increase the basal metabolic rate as much as some forms of weight-training, and may therefore be less effective at reducing obesity. However, this form of exercise also allows for longer, more frequent activity and consumes more energy when the individual is active. In addition, the metabolic activity of an individual is heightened for several hours following a bout of aerobic activity.
Aerobic activity is also used by individuals with anorexia as a means of suppressing appetite, since aerobic exercise increases glucose and fatty acids in the blood by stimulating tissues to release their energy stores. While there is some support for exercising while hungry as a means of tapping into fat stores, most evidence is equivocal. In addition, performance can be impaired by lack of nutrients, which can impair training effects.
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Need helpful info about Cardio 
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