Hi again. Just to update the situation. I feel much better now since I stopped intervals, tempos, hills and fartleks. Shame because it was fun and thrilling.
Basically I performed one of the long runs, a half marathon, with a nagging feeling in the achilles area, but I didn't think it was going to get worse than that, so I kept on running. Immediatly after I finished the run the pain seemed to increase but it never got as bad as when I suffered it for the first time. That time I was limping for the next hours after suffering the injury, to give you an idea.
After that long run I left two whole days rest and did just a couple of short runs of about 4 miles before the next long run, which was a mere 12 miler at a very gentle pace. I finished it feeling almost OK, but it was evident that the culprit, as you guys rightly point out was overtraining. My next long run will be this Sunday and I'm pretty sure it will be just fine...hopefully. Steep tracks seem to bring back the injury, so I'll be avoiding them. One thing I find curious is that the end of this Achilles Tendonitis is marked also by the dissapearance of the stiffness I felt in both Gastrocnemius. I'm sure there's a strong relation between both there.
Many thanks to you all.