Just keep calling the attention from wandering over and over again. Tough it out. Full instructions on how to do this are in Chapters 7 and 8. Then you can start pumping your energy into mindfulness. Do not, however, go so far with concentration that you find yourself going into a stupor.
Mindfulness still is the more important of the two components.
It should be built as soon as you comfortably can do so. Mindfulness provides the needed foundation for the subsequent development of deeper concentration. Most blunders in this area of balance will correct themselves in time. Right concentration develops naturally in the wake of strong mindfulness. The more you develop the noticing factor, the quicker you will notice the distraction and the quicker you will pull out of it and return to the formal object of attention.