calves are awkward things and grumble when overused and at unusual times
usually a symptom of something else not quite right and if you are marathon training this is likely.
check out your hip flexors by doing common stretches for these and do some glute strengthening and quad stretches
also in your training try and conciously ease back on stride length and strike the foot with a midsole rather than forefront of foot (i'd put money on you being more forefoot striker) for a while until the tightness eases.
you can roll and stretch your calves all you like and this is good practice but it won't solve the root cause of the issue.
given its only a month away avoid pulling calf at all costs as if youre up to 18 you'll have done almost enough work.
have similar issues and whilst jury is out on sports tape (kineaseiology something or other) it has really helped me. You will find that your calf will be 90% in your mind now and the tape helps provide support and give you some reassurance.
