I am not sure I understand your route. If I were coming from Detroit, I would head to Ft. Collins, find a low point, cross the divide, head to Walden then Kremmling then to a point north of Vail that looks low and cross over to Vail. Get some old sectionals and a yard stick and lay the route out adance (absolutely required). 12.5K ought to be enough if you plan it right. Have plenty of gas, do not push the weather, know how to properly lean the engine for takeoff, be knowledgeable about density altitude, navigate accurately, watch out for turbulence and downsloping winds. Flying the mountains is not hard but it does require a lot of specialized knowledge. A better way to do the trip is to leave a day or two early and stop on the front range and get a mountain checkout from a competent CFI. You should also call the FBO at Eagle and get an estimate of cost before you arrive. BTW no IFR without a lot of mountain experience and Oxygen.