"The long haul" means the same thing as "long-term," "the long run;" it means an extended period of time, probably many years, and, often, a sustained effort. Here, it means that "better weight control, exercise and a healthful diet" won't work an instant miracle, and you might not see their effect in a few weeks or even a few months; but if you keep it up as a regular habit you might live longer and be healthier in your later years.

It means for a long period of time and implies there could be difficulty involved.