Percentage of Lactate Threshold for a marathon?
Nowhere near your lactate threshold. Marathon running should be entirely aerobic (on race day, of course, I'm not referring to all of your workouts). The general rule of thumb, especially in the early miles (it's really easy to go out too fast), is that you should be able to talk to someone running beside you without much effort. Late in the run (the last 10 miles or so), you should be able to speak in short statements, though it will be harder. You've got to keep your pace under control early on. If not, even if you're well trained for the race, you'll suffer badly in the second half.