Originally Posted by mev

Following Adventure Cycling routes should be ~2190 miles from El Paso to St Augustine + 750 miles to DC or 2940 miles.

The occasional cutoffs of Adventure Cycling route I took was 250 miles shorter from El Paso to St Augustine and it would make sense to go via Jacksonville rather than cutting further south, so I'd say roughly following this route with occasional choices would be closer to 2600 miles.

Leaving El Paso on December 17th and arriving in DC on January 19th is 34 days so we're talking just slightly more than 75 miles per day, a little more if you factor in a rest day or contingency day or two. Going a little further north might save a few more miles but also increase the number of hills you go over and increase risks of inclement weather.

Everyone is going to look at things a little differently, but if it were me, I'd keep along the rough original southern plan with an occasional cutoff and keep in mind cities with Amtrak baggage service that aren't too far away as a backup plan to go either forward or back if needed in a contingency because time runs short or other reasons.