remember with a walking foot you need your feed-dogs to be up (engaged) if using the darning/free motion quilting foot then the feed dogs should be down.
distributing the weight of the quilt is important to keep drag down and i nice even speed. you should not have to 'pull' on the quilt to get it to travel along...it should be inching along with just some guidance to keep it straight.
start in the center, go down to edge then turn quilt go back to center and travel in the new direction to edge.
it helps me to do one quilting line the full length then turn again to start the next one. starting in the center instead of the outer edge helps keep the (pulling out of shape) to a minimum.
practice, practice...