It takes two days--about 45 hours (40 returning due to shorter connections)--to make this trip, which is about 1400 miles. According to Google Maps it would take about 21 hours to drive the same route, or about 19 days to walk it...
The exciting news this time was that the trains ran on time! In the past the Zephyr has typically been four hours or so late getting from San Francisco to Denver, but with reduced freight traffic (and a recent lawsuit between Amtrak and the Union Pacific regarding shared use of the UP tracks), they seem to have reached a timetable that works.
