This is meant to be a brief guide to help you understand how to play Hecarim. In my opinion, Hecarim's strength lies in the fact that with high movement speed his early game ganks are ridiculously strong. To this end, I start boots / 3 and jungle until I can upgrade my boots to Boots of Swiftness, giving my level 4 game extreme power. With such high movespeed, along with your Devastating Charge, and Ghost, getting away from Hecarim should be a challenge. I choose Swiftness Boots over Mobility because it is much more important to be able to continuously move fast rather than losing the boost after being put into combat.

I then build Wriggles because his mid game jungle clear speed is mediocre because of the 50% reduced damage on his Q. Then, I build Trinity Force because it gives everything Hecarim could possibly want: Movespeed which turns into damage, A Sheen proc on every Q, which is on an extremely short cooldown, and a slow on his auto attacks. After Trinity Force, I build defensive items based on what I am facing. I like Sunfire Cape since most teams have a lot of AD, but against an AP team FoN is great as well.

Lastly, as for Summoners Ghost is extremely important for Hecarim because of the bonus damage caused by movement speed, and while he can initiate with his ult, closing with his Devastating Charge is very important. And Smite is an obvious choice because Hecarim is a jungler.

As for Runes and Masteries, these are my preferences for Jungling and Masteries, if you feel that another set is better then by all means use it. I like these because the AD benefits his Q damage, and his auto attacks. Armorpen would also work well, and movement speed Quints would work as well.

A note on Lane Hecarim: Hecarim works very well as a jungler because of his initation of ganks, however in lane he isn't as powerful because of the limited heal from his W, and his inability to trade effectively. In order to trade hits in lane, he has to hit his Q more than once, which isn't reasonable, especially against champions like Olaf that hit with their one ability then run away quickly. Therefore, I don't suggest using Hecarim as a lane champion, as Top Lane. Likewise all of his abilities passively push the lane, which is not a good thing since you will end up overextending and either losing CS or getting ganked.