I think that the best answer is that since the iron man has an ARC reactor which basically is a super power source which never runs out, well it does but after approximately 400-600 years, well beyond the human life expectancy. Now in his boots, tony stark has some sort of very powerful rocket engine which seems to last for ever, this is because the rocket is a VASIMR engine or something equivalent to it in the marvel world.
You can see how the VASIMR works on Variable Specific Impulse Magnetoplasma Rocket (Wiki)
This type of engine can use any gas as fuel and act as either a rocket or ion thruster enabling very high efficiency, simply such a highly technologically advanced suit can simply extract hydrogen from the air by pressurizing and condensing rammed air thus giving water, then through electrolysis hydrogen is used to power the equivalent VASIMR engine, and since oxygen is also produced, it can be used to supply oxygen to the suit's user, enabling the suit to fly incredibly high where the atmosphere is thin enough to cause minimal drag yet provide enough water vapor for the suit, in case the suit needs a quick refill, it can simply descend to lower altitudes where the water vapor content is high, by flying at high altitudes it is able to achieve hyper sonic speeds of mach 5 (3840mph) since drag is very low. Also it can use ambient air to mix nitrogen with oxygen to get the 78%-21% mix which humans need to breathe properly at approximately 14 psi of pressure so that the suit remains pressurized.