How to drop from 2:30/100m to sub 2 min/100m over >4-5k?

First of all: I have no idea what I'm doing. I'm a lap swimmer and do it for fun. So in open water I'm a 2:30/100m swimmer for Ironman/5k distances and back up to doing 10-12km a week in the pool with no real training plan other than a bunch of 300's to 600's until I get bored and crave coffee. However, I did make a sizable bet with a friend of mine about who'd be the first out of the water in an Ironman later this year, so here I am. So this is what I need help on: 1) Anyone you can recommend in the Chicago area who will look at my stroke and make corrections to make sure my mechanics are ok? 2) Is this even possible? TIA, NC