Given what you've described, I would recommend a regular yoga or pilates practice with an instructor(s) who really understands anatomy. Stretching will presumably help with some of your flexibility issues, but I don't think it will help you strengthen your core and/or deal with your muscular imbalances in the way that a regular (and by regular, I mean 2-3 classes a week) practice will. I started yoga at about the same time I started riding and it's worked wonders for my back, abs, hamstrings and hips (although they remain my bugaboo). It's a very good counterbalance to cycling.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher