I really like doing yoga, or stretches of all kinds, especially on my rest days, for a few reasons.
1. You can get acquainted with your body in a different way, more aware of what's tight, what's achy, what's balanced or imbalanced. This will help you programming the rest of your training and also being aware of those tender places.
2. Flexibility becomes more and more important as you age. (Since I'm in my fifties, I've had to pay more attention to this!) You can identify "old" (unconditioned, old) people because they walk weak but also because they walk stiff. A youthful body is strong AND flexible.
3. It helps develop your athletic focus, promotes relaxation, and really helps with alignment and posture.
There are other benefits, I'm sure, but these are the ones that leap out at me. I like taking classes, myself, but over the years I've learned enough to do the basics myself. Before my surgery, I did the sun salutation, a warrior pose or two, and some spinal twists every day. I'm looking forward to getting back to that little by little as I heal.
Warrior poses, downward-facing dog, triangle poses, seated toe-touches, are all good ones to try for calves and ankles.
Those who hear not the music, think the dancers mad.