I have been doing yoga regularly for my entire adult life. In my opinion it is extremely important to have an experienced yoga practitioner investigate your form and help you tailor positions and transitions to your comfort/flexibility level, in a way that responds to any injuries/problem areas in a constructive way. I taught my boyfriend who is a very fit backpack hunter a simple modified Vinyasa sun salutation- it illuminated some latent injuries, inflexibility/tightness in hip flexors and lower back...the kind of things that some day become chronic pain problems.
A good rule of thumb for yoga: if it hurts (in a bad way, as in it is painful), don't do it.