Colin - what stretching are you doing after your runs? If none then start standing on edge of stairs in your bare/sock feet and hanging your heels off back of stair.
There are lots of variations on this but gives your calf a good stretch and the lifts also strengthen your achilles which will be the next part of you to hurt if you are getting tight calves.
I do 20 - 30 secs on one leg just letting my heel hang off then swap legs and repeat twice then i do 20 lifts up on to my toes on each leg and repeat three times a day.
There are other calf stretches you should be doing too but this is the one I do all the time.
Good luck with your marathon.
Cheers, Skinny.