I also suffered from this - every time I hit about 6km my right foot would start to go completely numb until I couldn't feel it any more. I went and saw a physio and along with the calf stretches already mentioned I was shown a stretch similar to a hamstring stretch (i.e touching your toes but sat down with your feet against the wall) but you slouch your back and put your chin on your chest. Really push down (yes it hurts!!) and do four reps and hold for a count of seven. After a couple of nights I was pins and needles free!!