The fact you had an MI and your EF (estimate) is slightly below the normal range (55-75%) should not be any decisive factor in life expectency. If you had a heart attack, it did very little damage if any.
What is determinant for longivity is the underlying cause and how successful is the medically treatment, genetics, heart healthy diet and lifestyle, etc. The probability is high that you will live another 20 yerars or more if determinants are favorable.
As regards heart condition you will live as long as any normal person. You need to take due care, as you have mentioned. Follow healthy lifestyle, take your medicines, control your diet and regularly walk/exercise as instructed by doctors. Go for necessary regular checkups. I am not a doctor but had suffered from MI and have a stent as well, with much lower EF.
Good Luck.