Woody, just take the VO2max test and find out your true LT. First start training around one key workout a week around your LT and with plenty of rest. A few months later add another key workout. All this time keep increasing the distance in your long workout per week.
If you haven't trained before, 5-6 hours a week is plenty. Start with 3-4 hours and increase to 6-7 hours, up to 300 hours a year. If you can make it through the first year without injury then maybe go 350 hours the next year, if not, 330 hours. You get the pic?