Slow and steady yoga postures wins the race

The ancient yogic text, Patanjali Yoga Sutra, defines yoga posture (asana) as SthiraSukhamAsanam. Do only as much as you comfortably can and then just stretch a bit more (to improve body flexibility). Use the breath as a reference point, when it is light and long, then the muscles begin to relax, but when it is jagged or uneven, it means you have over-exerted. Going slightly beyond your comfort zone will keep the yoga practice interesting and will add a spark of challenge as you progress and adopt new postures.