HIIT vs. Aerobic for building base miles?

My training partner is attempting to drop some extra pounds between now and when we start training for next year's marathon (December 31, totally coincidentally - the marathon is in early May). We've got 11 weeks. I know he can continue to lose during training, but I'd like to have a good idea of what to expect going in to it (I'm the one designing most of our training programs, though it's the blind leading the blind). I'm wondering what the balance between HIIT and aerobic miles should be to put us both in good shape.

Back story - We've worked up to a very comfortable 20-30 miles a week base for the past 6 months, typically with one interval run, one long slow distance, and then 2 easy runs (I admit, I sneak a 5th or 6th run in, but he lifts weights on two off days - we both rest one day a week). We spent the summer training up for a half marathon, which we rocked last weekend (WOOT!), absolutely no troubles whatsoever. So now we're back to building more base, around 20-30 miles a week again.

My gut says 2 easy, slow aerobic runs (~4 miles each), 2 HIIT runs (~3 miles each, usually fartleks or repeats or hills), and one long slow distance (~6-10 miles, depending on our route). But my gut has no science in it, it's just trying to find a balance between the two pieces of contradictory and most common advice I've heard - "Build a solid, slow aerobic base" and "HIIT will drop weight and improve fitness fast".

What do you guys say? I'm pretty sure no matter which way he tries to go, he'll drop weight just by putting miles on his shoes, he's done it so far quite admirably. But we'd like to optimize it if possible. Is it a balance, or always a win-win?