RAP appears to be a 100% non-mainline city these days; I thought UA ran some TEDdies there last summer, but not this summer apparently.

I'd say it's more like 7 hours by car DEN-RAP.

One possible alternative would be to fly ATL-FSD and then drive to RAP. Much shorter distance and all freeway via I-90. DL does have a nonstop ATL-FSD although (of course) it's an RJ. You'll have to check pricing - probably high.

Now that I know it's business I think it's probably slightly more cost effective to fly ATL-DEN-RAP on UA/UX. Knowing DL's icky website, be sure to check manual multi-city itineraries ATL-MSP-RAP and maybe even ATL-SLC-RAP.