McKenna was arrested in hospital while being treated for frostbite after the attack. He had earlier pleaded guilty to wounding with intent, but in July was found guilty of attempted murder.
Sentencing him at Swansea Crown Court on Friday, Judge Paul Thomas KC described the attack as "wholly calculated" and said McKenna had left Sidaway to die in the bushes.
Sidaway was walking her dog when she was attacked. She screamed as McKenna stabbed her repeatedly. The attack was reported by a member of the public, which prompted a large-scale search by Dyfed-Powys Police.
McKenna, from Carmarthen, hid in woodland for three days without food or water before being found and taken to hospital, where he was arrested.
The BBC reported that the judge found the attack was premeditated, with McKenna lying in wait for his former partner.