Speed work and a long swim will be your best bet for improving speed and endurance in the water. I would definitely work this out more with your coach, but these were the basics of my IM swim plan:
3x/week, with one moderate, one speed, and one long swim
Moderate swim was generally 2500-4500m, usually with a laddered main set i.e. 100, 200, 300, 400, 300, 200, 100, with a few 100 and 200 drills thrown in, WU and CD.
Speed set always had a 16 or 20x25, mainly on 5-15" rest. Then something along the lines of
(3x100 build, 1x150 easy
) x 6 as the main set. Also a few drills, WU and CD.
My long swims ranged from 4000-6000m, with the main set centered around 400, 600 or 800m repeats, occasionally alternating in paddles. Usually these were done on 45" or 1' rest in between.
Hopefully this provides you some sort of vague outline, or at least a few ideas.
One thing, though - you mentioned you want to keep most of your swims to 30-40 minutes. Depending on the distances you're trying to do regularly, that may work. Though I'd be hard pressed to say that you can fit in a 2 mile swim in 40 minutes...if so, you are definitely one heckuva fish.