I try not to get too hung up on 'fat burning' versus 'fitness' as it clouds my thinking. I concentrate on burning the most calories per session. If your goal is to burn say 500 cals and you can do so in half an hour then you have completed your workout. If, however, it takes you an hour then so be it. You've still burned 500 cals.

I understand that HIT assists with continuing to burn cals after completion.