The main problem with trying to walk is the lack of pedestrian-friendly infrastructure. There is a pavement going north on the Western Perimeter Way that becomes a cycle path / buses only crossing the river which explicitly excludes pedestrians, so getting to Longford Old Bath Road is not legally possible. A pavement of sorts continues around the Western Perimeter (inside the fence) and may continue on to Bath Road, but I suspect you might get stopped by security before you got there. To the south there is a shared use path (cycle plus pedestrians) along the Western Perimeter road to the Southern Perimeter road and Stanwell, but the 'pavement' in other directions is patchy.
I personally have no issues catching the 423 or 350 bus within the free travel zone to Premier Inn or Sheraton and then walk up to Harmondsworth and the greenery around Waterside if you want to stretch your legs.