The data sheet for that heat pad gives zero information as to current demand.
You need to advise what current it draws when you apply 5 volts to it. From there you can calculate power consumption and then determination of what battery capacity you need to achieve the desired running time.
Have you considered using 3.4 volt lithium cells and doing away with the step-down regulator. You also need some form of temperature regulator as the data 'curve' given suggests the pad keeps getting hot until it reaches a delta-T of around 40+ degC which is way above the temperature you require.
A single simple wire-wound power resistor could also be used as a heater module as all resistive power drop is expended as heat.