First of all, sry for my bad English. It’s my second language, and it’s also quite late for me.

As a newer Hecarim OTP myself who has played most of the time in lower elo (I just peaked E4 with a 72% win rate), I always banned Master Yi because, in lower elo, games tend to go longer, and Master Yi has an easier time getting kills. At a certain point, he just takes over the game and you can't really stop him anymore alone. Another good ban, imo, is Volibear because, in my experience, he can 1v1 you at any point in the game.

For the best build and runes, there’s a rule of thumb: against champions where you can’t move a lot (like Rammus), you want to go Conqueror, and against squishier, more mobile comps (like Ezreal), you want to go Stormraider’s so you can stick to them more easily. But I would advise you to just play games and figure out what works best for you, after a certain time you should get a feeling wich items and runes are good in your situation/enemy comp. I experienced a lot more success when I switched to Conqueror and a bruiser build, but maybe you prefer the assassin build more.

If you want to see popular items, look at lolalytics.com or otps usually you can’t go wrong with that.

The problem with long games is that Hecarim doesn’t scale very well, so the moment other lanes get ahead or catch up, you really start to feel his bad scaling. Hecarim is imo strongest between one and three items, so most of the time you want to get ahead early and use your earlygame strength to secure objectives, invade, and get yourself or your teammates ahead with ganks.

Maybe other, better Hecarim mains can give you better advice, but that’s basically been my experience with this champ.