It totally diables the FAST TRAVEL, but allow you to fast travel only when you are riding a horse.
Sorry if this sounds noob...
http://www.gamesas.com/index.php?/topic/1109315-relz-map-marker-overhaul/ seems to have an option close to this. Not using it myself so not sure how it can be configured:
If Fast Travel is enabled, you can set up MMO to only allow Fast Travel for shorter distances, or for a limited amount/distance within a certain time. You can also set this limited fast travel to allow longer fast travel if you are on a horse or have high Endurance and Athletics stats.
EDIT: Yes, it seems you can set it up to be exactly as you want, e.g. disable fast travel if on foot (or close to at least if you set it to 1 cell) but enable it if riding:
; Set maximum allowed distance to use Fast Travel. The distance number of cells, where Chorrol-IC is ca 25 cells.
; Set to 0 to allow unlimited distances.
set tnoMMO.fastTravelmaxSingleDistance to 0
; Set how much longer distances that are allowed if riding. Set to 1 or less to ignore riding status.
set tnoMMO.fastTravelRideFactor to 2