The "fat burning zone," 50-60% of MaxHR, is really something of a myth. It has been perpetuated to encourage people who don't like to sweat to at least get some easy-to-moderate exercise. Having said that, the zone from 60-70% can be very beneficial for burning fat. You don't burn fat [or, more accurately, convert fat into glycogen] in that zone any faster than in higher aerobic intensities [say, 75-85%], but you don't burn it that much slower, either, and it's an intensity level that most relatively fit people feel comfortable maintaining for extended periods.