eni mevish , 20 , was stabbed to death by david marshall , 68 , last november .
the pair had become friends six months earlier while eni was out jogging .
he stabbed her at her home in the lungs , liver and heart with kitchen knife .
marshall , who previously abused a 14-year-old girl , admitted murder .
he must spend at least 20 years behind bars before applying for parole .