I changed my L3 to assist. (I main healer)
R3 you can change to macro #99.
A simple Sprint macro is just
/ac Sprint
A more complex Sprint macro is
/ac sprint <t>
/mount "Main Mounts name"
/mount "Backup Mounts name"
/ac Sprint
[if the company Chocobo is not your main mount place it in the backup slot. If it is your main mount put your runner up, for when your choco is stabled]
What this does is:
[*]It lets you Sprint towards something without attempting to mount if you target anything, even yourself.
[*]It will have you mount if Sprint is on CD or if you're not targeting anything.
[*]It will summon your chocobo companion as a mount instead of your default, so you don't get an error while he is summoned for combat.
[*]It will also summon your backup mount when your chocobo is stabled, if he is your default mount.
[*]Finally, Sprint is repeated at the end so that if you are In a place where your mount is forbidden and you aren't targeting anything, it will have you Sprint.

Figuring this out has made my hotbar much more enjoyable.