Sounds like a problem I get now and then which I thought was achilles tendonitis or the start of it but the pain seemed to be more on the back of the heel rather than the actual achilles. I think it sounds like achilles bursitis... google that and have a read, see what you think.
Stretching my calves seems to have helped me a bit (both parts of the calf muscle).
*Please note that I have no medical qualifications, just years of struggling with various running injuries / niggles!
The tunnel is still closed. Apparently due to a landslip of the embankment on the south side of the tunnel (or Tipton side). I think you can get through the tunnel on the towpath on the otherside of the canal but then the towpath ends and you have to leave the canal to get back over to the towpath on the other side. There is an easier running diversion using the path which runs alongside the fence of the Coseley School which brings you out at the same place but I'm sure the organisers know what they're doing.. well you'd hope so!
That route always gives strange GPS results as well because of a few canal tunnels so take your Garmin results with a pinch of salt. There's a tunnel around the Smethwick area which I train through and my Garmin 410 quite often gives some strange results.