As long as you are fit and healthy - you have nothing to worry about. I would suggest that as you are reasonably fit you may have a low resting heart rate, in turn this may mean you have slightly low blood pressure, when upon standing could cause a sudden rush of blood from your head, making you feel like you're about to lose consciousness. When this happens, slowly sit or lie down and the problem should resolve itself within a few minutes.
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If you have recently started doing more exercise than usual this could be the cause, however I would suggest you consult your GP as it could be an underlying cardiac problem, such as a heart block or a build of fatty deposits in your arteries.
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Well, there are many things that make for a healthy heart, such as: