The fastest way of course is with a Chocobo. :)
Due to my limited knowledge, I must admit, I don't know of a coupon.
In the case that you are under level 20, and do not yet have a Chocobo, of course, you'll have to walk. Personally, I play this game very conservatively, but if you're feeling brave, you can try this route:
Bastok Markets >> S Gustaburg >> N Gustaburg >> Konschtat Highlands >> Valkurm Dunes >> La Theine Plateau >> West Ronfaure >> San D'Oria South
I'm sure there are a lot of brave souls out there that have done this at levels under 15. I'm not one of them.
I wish you safe travels.