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.
I did it last year and a few years back when it was the Hinckley Half. If I remember rightly the first 8 miles or so are fairly undulating country lanes, nothing particularly steep or hilly. A bit of a climb up from the start to the top of a canal bridge then I think it's mainly downhill and quick afor the next few miles. Then there are 2 fairly significant climbs towards the end. I think one is at about mile 8 and the last one is about mile 11. Not steep but quite long uphill drags. One is known locally as the "Gated Road" which climbs up to Market Bosworth.
Quite a nice scenic course and always seems well organised.