The heart is a muscle—the muscle that keeps the other muscles going. It's good to build and tone our muscles, but the essential thing is doing whatever keeps the heart strong.
With our spiritual heart, we strengthen and tone the heart through the Word of truth by receiving its message of God's goodness and grace. Keeping our spiritual heart strong and fit must be our first priority, the thing we do above others.
Paul would agree: "Train yourself for godliness; for while bodily training is of some value, godliness is of value in every way, as it holds promise for the present life and also for the life to come." God's training designed to grow us in faith.
Lord, may I feed on Your goodness every day, so my heart will grow stronger through the Spirit.