Leg weakness and fast lactic acid buildup/burn

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I love cycling heaps, but I'm finding that I tire easily, my legs feel weak and seem to develop lactic acid buildup extremely quickly. I warm up well and even happens when I'm not going particularly fast. It is terribly depressing. It seems all the hard work I've done previously has gone down the plughole. I have recently been diagnosed with hypothyroidism and am on thyroid replacement and my levels are still up. I read that hypothyroidism can cause muscle weakness - has anyone experienced this?
Also, don't laugh ;-) I've got short legs being a tiny person of 150cm and wonder if having short legs is a disadvantage as opposed to long slim legs? Anything I can do? (lengthing of legs not an option)http://www.cyclingforums.com/newthread.php?do=newthread&f=27#
Thanks for your help
Regards, Sam