Core training within the yoga tradition doesn't focus on the muscles contracting; the emphasis is on openness. Instead of putting your attention on what's being tightened or clamped down, consider the places on your body that are allowed to stretch in new ways.
The challenge? Do 15 minutes of asanas (yoga poses) each day. Want more? Spend an additional 15 minutes meditating and eat at least one vegetarian meal per day. It sounded pleasant, light. But would 15 minutes do anything for me?