As someone noted above I would also recommend a west to east route. Out of Denver you start climbing almost right away. Starting out of Glenwood Springs though I think you can take 3 easy days with a steady but never steep or long climb to Vail. Another reason is I think it better to go over Loveland Pass (US 6) west to east; its 32 miles between hotels in Dillon and Georgetown and 3,160; climb west to east and 3,760' east to west. Most of the time you don't really notice the interstate and most cars take the interstate as its the fastest way for them. Will have some "local" travel on the parallel road but I don't think it terrible. Are you thinking of taking the train from Denver and Glenwood Springs (returning to starting point); I ask because I am pretty sure bicycles can be taken in baggage car without being in a box on that route.