to burn fat...
u should follow a formula to calculate yr min. and max. heart rate when doing exercise.
in general case, most fitting centre they have a chart on each machine (e..g walking machine, running machine or sky walker)
then u should check yr suitable heart rate in yr age rate. e.g. if u are aged 25 and from chart u may check yr min. heart rate 120 and max. is 180.
Then to burn fat, u should keep yr heart rate in between these figure (normally ~ 140-150) and continue the exercise in 30 minutes...then u are really can get the result of burn fat