5V x 0.7A = 3.5 watts
9V x 1.2AH = 10.8WattHours
So the very best you could ever hope to achieve with 100% efficiency = 10.8 / 3.5 = 3hours
Assume say 50% efficiency then run time might be in excess of 1 hour
If you are only getting a few minutes and your battery is getting hot then I suggest you either have a heat pad that consumes more than 700mA or you have a serious wiring error.