training for flat on hills/trails

I recently moved to colorado from florida and have been thinking about future race goals!! Since moving i've been doing ~1 trail run and 2 road runs per week, with an additional long run alternating between road & a trail run or hike. a lot of the marathons i'm looking at are fairly flat and of course paved. For my 2-3 longest runs I could drive to a pretty flat paved route, but I was wondering if you had any advice on how to balance the rest of my long runs. I want to make sure I have enough practice similar to race conditions while still exploring all that co has to offer. How often is it okay to replace a long run with a trail run or even hike? 

For some background I ran NY last year and have done lots of halfs/15ks. I typically run 10-12 mile long runs when not training so it's more the 16-22 that would be outside of my typical volume.