long ride power

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I know I have relatively good endurance compared to my sprint and short term power but how long should it be possible to maintain level 3/tempo power? I did my first very long ride this week - 5 hrs, i ahve done a few 4hour rides so far this winter but nothing as long. for this ride my ave power was 213w and np 229w, accordign to CP my tempo range is 213-254w. FTP is from a critical power test on a Lode ergometer. should it be possible to put out that sort of power for that long?