Body is not stiff, mind is stiff.
Yoga is your mind control capacity.
Do your practice and all is coming.
It is very important to understand yoga philosophy: without philosophy, practice is not good, and yoga practice is the starting place for yoga philosophy. Mixing both is actually the best.
Yoga is 99% practice and 1% theory.
The full ashtanga system practiced with devotion leads to freedom within one's heart.