Flat land speed development
You have a specific training question that your coach ought to be able to answer for you, and I will not address that here. A larger general issue is one that I will address, since it is of wider interest. 

Your general concern is that you won't be able to simulate race conditions in training, especially long steady output. However, speed is determined by power and drag and you can work on each piece even where you are. A rising tide lifts all boats so the power developed while climbing hills and during intervals will mostly be transferable to longer steadier output, especially if that output is substantially below functional threshold power (as it will be for most half- or full-IM efforts). Similarly, you can work on positioning to minimize drag, on your bike handling in rolling terrain (unless the slopes are too steep and the roads too technical), and changes to minimize rolling resistance. 

So, the general answer to your general question is that one can separately evaluate and train the components that make one go fast even if training conditions are different from racing conditions. It just takes a way to measure and evaluate progress in the separate pieces, so you can "translate" variable terrain performance to flat terrain performance. 

In your particular situation, look toward the bottom of this bike.SE answer to see an example of time splits for an IM. Your swim and run times put you closer to the front of the pack than the middle, but you have lots of room to improve in your bike split. Your run and swim splits suggest that low power isn't your particular issue -- though you can always work on more. Your bike split time suggests that aero drag is likely to be your issue. Your coach can help you work on aero drag even on uneven terrain. To measure and evaluate improvements both in power and in drag may require some additional instrumentation.