I had achilles tendonitis last year in my right achilles. I know what you're going through.....the pain can be terrible, shocking first thing in the morning, you think you'll never walk properly again.....let alone run. I sought help through an NHS physio who recommended daily eccentric exercises, calf stretches, towel crawling with the ball of the foot and 20 min icing each evening (bag of frozen peas!):
I've goodled 'eccentric exercises for achilles tendonitis' and there are dozens of articles that will help you. They really do help, bent knee and straight leg, 15 reps per leg twice a day.
The achilles tendonitis came on in April 2011 and pretty much plagued the rest of the year, but it didn't stop me still getting some PBs after a couple of months of physio, I managed a 10K PB in July 2011. Strangely, even when it was at it's worst and I was limping everywhere I still managed to run a bit after huge warmups (consisting of lots of calf stretching and actual eccentric calf drops right before training), I used a cycle machine to keep fitness up too. Many people give up running, I didn't......and that is my recommendation, but we are all different!
It probably took 6 months to get rid of all perception of symptoms.......but no kidding, I don't believe that anyone could have had a worse case of it than me (in terms of pain anyway, my achilles was swollen up to near as dammit an inch wide!) so please don't think that I had a mild case. My physio never said to stop running though, which i was grateful of.
The cause may well have been poor form, I used to be a heel striker and am working hard to banish it from my running.......when I'm exhausted, it can creep back though.
I still do my eccentric exercises everyday. They should be considered vital for anyone who runs whether they have had achilles tendonitis or not.