Yoga?

I agree with above.  Check out instructors who will talk you through adjustments for your level. Know your limits.  Yoga is not about getting the poses perfect as much as it is connecting with your body and releasing the tension.  Because of running, hamstrings are very tight.  Do not expect to get your heels to the floor when everyone else can.  Don't try.  Listen to your body and let it go as far as it will.  Do not push it past its natural stopping point.  As you do more and more you will get more flexible.  Remember that yoga is not a competition.  There is no judgement, only guidance.   This is about you and your body.  The flexibility will come. 

I do yoga with my wife using DVD's.  I have used several ones, some specific to certain sports, some not.  I really think a lot of this is marketing.  Basic yoga positions; downward dog, sun salutations, upward dog, and a host of others I don't know the name of will accomplish your goals of increasing flexibility and strength.  A triathlete needs flexibility everywhere since we swim, bike and run.  I have several Rodney Yee dvd's and a 10 minute solution DVD I take with me on the road.  Rodney took a little getting used to, but he does a good job of teaching what you should be feeling as you go through a routine.  If I stick with it, I see noticeable increases in flexibility.  Most only take 20-30 minutes.

That being said, to me the most important feature is that you can do the moves they are teaching, you understand what they are trying to accomplish, and you like watching/listening to them.