13 songs isn't that short for Sabbath. Even during the 70s they would play around that many. Considering Sabbath was in a rebuilding stage at this time and didn't have a lot of fan or record label support it's no surprise they only played for that long.
I've seen a bootleg video from this tour and Tony Martin sounded great. He did an awesome job with the Dio material. I've always loved how good he sang COTS.