Wrap yourself in foil to keep warm

It has always seemed strange to me that you can keep yourself warm with just a thin piece of foil but that's exactly what a lot of runners do. It shows that we must lose a ot of our heat as infra-red. Suppose that I am 2 metres high by 0.5 metres wide. That might give a total area of 2 square metres front and back. By Stefan's Law where P = sigma x surface area x absolute temperature^4, and with 37 Celsius being 310K I get an output power of 1.1 kW. Googling suggests other people are getting 100W.