It is possible. I've done 4 in a row in my Kona prep. You don't sandbag it just gets harder. Imwas hitting my target heart rate in every video. There are only a few all out sessions on the videos other than revolver and downward spiral and they relatively short bursts. Its like being on the verge of throwing up for 3 1/2 hours.
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Check out my blog
http://www.ironscottallen.blogspot.com
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Check out my blog
http://www.ironscottallen.blogspot.com