Thanks... Well the latest on this is that my physio advised me to change my running style. Apparently all of the impact at the mo is going through my achilles, I'm not using my glutes properly, and the other muscles in my lower legs (I forget their names, the ones that run up the inside and outside from your ankles) are not strong enough. All of which means my feet are collapsing in - despite my stability shoes - hence the foot and shin pain. So no massive surprises there, which in a way was quite reassuring.
The good news is that I haven't injured myself enough to warrant stopping my training - phew - and he didn't even think that I need orthotics, just some exercises to strengthen certain muscles. He also showed me some ways of improving my running technique...
The bad news is I've been trying these techniques over the last week (16 mile run and 4 other runs between 4 and 8 miles each) and because the muscles I need aren't very strong at present I am finding it really hard. For example, last night I ran 4 miles... I felt like I was running flat out, and was panting as though I was running 8 minute miles (which for me is about as fast as it gets)... But when i got home my Garmin told me I had run an average of 9 minutes 40. Ouch.
So the question is - am I being stupid to think I can change my running style this close to the marathon? Or am I just being too impatient with myself - after all, it has only been a week...???