I'm struggling with the same issue. with the Merseyroads 24 exactly one month away.
I'm actually typing this sat at home with an ice pack on my leg!!
Went out on Saturday to do the Cat and Fiddle in Buxton and raised my saddle ~5mm as I'd been experiencing issues with my knees after hitting the hills and having the club coach lower my saddle. 120 miles in all
On Sunday it was a bit sore, so lowered the saddle again. Cue 90 more hilly miles and it was done for. Next morning I could hardly walk.
My strategy is Ice 3 times a day & ibuprofen to stop swelling. Then when I get back to the bike, perhaps next week, daily 1 hour sessions (possibly on turbo) avoiding any hills and getting out of the saddle. Make sure i do some gentle stretching after each of these sessions to keep the calf loose.
When i get back on the bike I'll be moving the cleat position as far back as it will go and lowering the saddle. This takes strain off the achilles when peddaling. (Did this quickly on sunday's ride and it didn't feel as different as i thought it would)
The other thing a physio has recommended was to keep a long towel next to the bed. When you wake up in the morning, pop the towel round your toes and very gently pull to stretch the achilles before getting up and walking on it. Sometimes those first few steps in the morning can undo all the repairing work done overnight.
Hope you notice some improvements soon. Regarding fitness, you may loose some of the higher intensity fitness you had, but the long miles stay in the legs a lot better so I wouldn't worry too much.