Yeah, did it myself. A fully bonded Arabian and whole ton of tonics is essential.

@Paddywoo98 Agreed, but it seems the most essential part that no one talks about is the stirrups, which grant massive permanent buffs to your horse. In this video I used a fully bonded Black Arabian (which is the best horse the player can legitimately achieve before the Epilogue) with the Hooded Stirrup, which not only adds a +2 bonus to both speed and acceleration, but also decreases its stamina drain rate by 50%. And of course, it always helps to have the Horse Care Package.

It's also worth noting that this challenge is best tackled before chapter 6, as Arthur having Tuberculosis and minimum weight makes him significantly more fragile, making an already difficult task near-impossible.

Agreed to all above, but a free Arabian can get broken up in Ambarino by Lake Isabella. That White Arabian is what I used throughout the game. Saved me a cool thousand but was a shame to lose her at the end.