Let me explain my thoughts. With the Swiftness quints: Hecarim need to faster then ANYone throughout the game. 1,5% movementspeed * 2= 3% movementsspeed and 3 AD. If we compare that to any other runes, like flat AD, u get 2,25 ad per quint but no movementspeed. My idea is to get the most value as possable for each rune!
Why flash, cuz when u use ur Espell those 4 seconds can be a bit hard to time'. Like if u are the engager (wich u are in most cases), or comes help from top to ur jungler. U want to hit with it! And use as much movement speed as possable to travel. The flash can secure u hit. And with 0/21/9 u actually dont need ghost really. I tryed ghost a couple times when i played Hecarim at first. But in most chases it was just for getting to drake from top, og gank other lanes. Ghost isnt just as viable in toplane like flash is.
Ignite is for a commen trade or reduce their healing.
Shurel is just a really well working item for Hecarim, works way better then ghost. I play Hecarim in teamranked, our support doesnt rush shurel (cuz i got it already). He need all the other stats alot.
the 1/21/8 is just a choice of taste i guess. He is manahungry if u want full potentiel. And the movementspeed is again for getting as much value as possable.

I hoped i answered some questions here:D
- Sorry for grammer and so on, aint my native language.

Uuuh... Not bad build. Well balanced Runes, I like this setup, giving you enough early power for farming and also some penetration. I don't like Runes of Swiftness (why everyone wants those 1-2,5AD from one -.-) and Shurelya. Shurelya is very good item but let support carry it for you and take something else instead. Also, why changing 10% Armor Penetration for another Movement? I would build 9/21/0.

Also, why flash? Ghost works much better, its better for chasing and while having ghost + E you don't need flash. Sometimes it would save you when this combo can't, but you will use ghost in much more ways and it scales with passive. It won't give you any huge AD burst, but about 11-28 AD is still nice enough. Ignite... OK, not bad choice at all but I would preffer teleport or exhaust on top. You are good chaser, Ignite isn't needed for you so much. Work on visual look of guide and I will maybe upvote later.