hamster

At what point do I stop seeing cardio benefits from increasing time at or above lactate threshold? Would 2 hours/week result in adequate gains (assuming that I pad my mileage to, say, 10 hours/week at lower intensity) or would I benefit from more? And what is the level where I'd need to worry about overtraining?

I'm a relative novice, I've been training since March and my total mileage for the year so far is about 1500 miles.