Originally Posted by corporate-wage-slave

It can be done, but it's 3 miles and much of it along the pretty soul-less Southern Perimeter Road. Hatton Cross to T4 is well under a mile and more do-able. If you were contemplating walking these sorts of distances then you'd be better off considering Hilton T5 instead, which is shorter, a more pleasant walk and takes you through an area with pubs and restaurants.