Just for the record, using Magnus stops the time sadly. Used T7 horse from the king to the Valencia stable, a speedy camel straight to Sand Grain Bazaar stable, then T8 with Sprint for the rest. Jumped off the side from Neruda Shen toward the gate to shave time but the rest was pretty straight line. Made it with like 3 sec to spare, I am quite sure you can do better using Map halfway through the desert to warp to Sand Grain and keep the rest the same but you have to be pretty close to not go to Valencia. I initially tried to see if the northern ground route might be better on the T8, it was at least 2 min longer than going straight across.

I did it a bit different. Used a +5 movement speed consumables all the time. Placed my camel at Valencia gates. Run from the king to the camel (because camel is slow at the normal roads), then took camel through the desert to Sand Grain Bazaar stable. Changed to my T7 horse with Sprint skill and Manos Riding Crop. Used Sprint all the time. If you have sufficient rank to use the Sprint automatically, be sure to turn it off/on around sharp turns (especially around Altinova) to not bump into the obstacles. Also jumped off the horse to get to Neruda and back. Then straight forward to Heidel, Velia and at Calpheon jumped off the horse at the bridge through the river, which divides the Calpheon. The autopath to Herman Feresio there is not optimal and goes around, but you can actually cut off a good portion of it, by running manually and taking the stairs to the palace, when the autopath goes left (see the Screenshots tab, red arrow shows the shortcut). Then double time to Rubin using the same stairs. Made it to him with 20 secs left.

To give yourself even more time, half way from Valencia to Sand Grain Bazaar you can use the Traveler's Map and teleport to Sand Grain Bazaar, when the it'll become available in the list of destinations.

I was able to successfully do this the "normal" way without any special trick. I used a T8 horse with 160.9 % speed and all skills learned with a saddle and a Manos Riding Crop +7.

I tested the quest twice, once by constant active riding over the north of the desert as well as straight through the desert, the result was roughly the same. I had some hiccups, but I arrived at Rubin with 25 secs to spare.

Done it with 8 minutes to spare, with no shortcut or maps. Took the long way around through gahaz. They've made it really easy so don't sweat it if you mess up.