Prepping for a flat tri in an very hilly area

I may be wrong, but I'd say the hills here can't do anything but help you. Climbing hills and getting better at them surely translates to faster flat speed somehow.

But, you're much more accomplished than I with your HIMs. I've merely gone as long as an Olympic.

Good luck!

That can be a hard transition to make.  I used to ride in an area that was ALL rolling hills.  We didn't have any long sustained climbs, but we also didn't have any roads that would be classified as 'flat' either...they just didn't exist!  My first century was chosen because it was flat (near the coast), so I thought it would be easy.  I was wrong!  It takes a lot more effort to keep up a continuous output when there is NO coasting.  My legs were wiped out by the end because they were so not used to it!

I would think that trainer rides would be a great idea because there is no coasting.  And if you ride outside, pick the least hilly rides you can find and work on not coasting.  Don't use the downhills to rest and that will help.   Try to avoid the 'interval' effect of the hills by keeping your efforts as steady as you can by backing off a bit on the uphill and attacking the downhill (safely, of course).

I would say break out the trainer. You might even get a better idea of a flat race from being on the trainer since you don't have to deal with stop signs and lights, traffic, etc. 

Hills can only help you be a stronger cyclist. I know that my speed on the flat races has greatly improved since I started cycling and I do mostly hills. 

That's a much nicer way than I was going to put it; what's with the coasting?? Pedal down those hills!

If they are too steep and you spin out regularly you may want to get in some trainer sessions to get used to the constant, steady, consistent cadence that flats give you.

I really like to do intervals on my trainer where sometimes I ride with a cadence of 105-115 and then at other times 50-70 cadence. I think drills like this at a constant wattage would be perfect for you. Either that, or find a flat area and do interval repeats along the same road over and over. All you need is a 1-2 mile stretch for that. Make the best with what you have got.

I agree with Fred and Brad.  Pedaling downhills will help but you probably want to spend a good amount of time on the trainer to get used to keeping the pressure on continuously throughout a ride.  Even if you push down a hill, it's hard to keep the same power output. 

^^ This ^^

I live in CO and much of my riding has hills.  Not huge hills, but I'm almost never riding flat. I've read in many places to try and keep power output steady anyway, which would include not coasting.  I don't have a power meter, so I just have to go by feel, but I basically never coast now.  This has helped my cycling tremendously vs. last year when I coasted down any hill I came across.

Learning to be the "constant cyclist" makes you so much stronger (well said Fred).  I did not even realize it until this year.  I've always lived in areas where hills were frequent, but moving to the coast where it is completely flat is so different.  You are always pedaling and it seems I always have a headwind.  (My calves are sooo much better now, haha).  While I do struggle with riding hills a little bit more now, the overall gains in my cycling have been superb.

So, to answer the question, you need to be pedaling on the downhills, not just a soft pedal either.  The trainer needs to be your friend too.  I never realized how much I coasted on my bike until I moved to the flat lands.