I was in a similar situation a few years ago before Loch Ness Marathon at a similar stage in training to you. Did the marathon in expected time by:
1. sticking to slow long runs (slow running was ok, just speed / hills that were bad)
2. all other mileage as aqua jogging in a deep pool
3. weekly / fortnightly physio on calves and Achilles
4. icing after every run
5. foam rolling
fitness stayed and themarathon went well with no Achilles pain. I then got greedy after the marathon and upped the speed / mileage and really messed it up but that's another story! After the marathon, rest and get it sorted properly.