Actually, as of now I haven't come across any spells that buff con saves, or effect endurance. I might consider modifying Longstrider to grant advantage on con saves to avoid exhaustion when performing a forced march (in addition to the 10' speed increase). Of course since the duration of...
First of all, I enjoyed reading the following thread for an overview of travel pace as defined in the PHB 182: D&D 5E - Mounted Tavel Pace
Disclaimer: all my info about horses and their abilities have been gleaned from internet searches. I have no actual experience or clue if anything I read is...