I mostly run roughly 8-10k distances, but have run 20k with a 500m height gain before (personal best). I can run at about 8 minutes to the mile at shorter distances, but have to slow it down above about 12k. I must admit, though, that I have not been out running for a few weeks.
Running can be tiring and painful, particularly during 'anaerobic threshold' training (http://en.wikipedia.org/wiki/Anaerobic_threshold) but it is well worth it, for the exhilaration of the endorphin rush and the satisfaction after running, and for the release of stress, tension and energy.
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"?I love not man the less, but Nature more.? - Byron
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I am an animal. Not normal is not bad. Question all. Life, liberty and the pursuit of happiness for all!