Hello everybody!
Before you start read this build, please note that I am hungarian, so sorry for the grammar. And please note I dont say this is the best build, and the only option. I just want to share my experiences and my build, 'cuz I tried it, and mostly it has worked. And dont forget, this is MY opinion.

I think I dont have to say any extreme about the runes. Runes for resistance and armor for tanks is always welcomed. Health is not as necesarry, 'cuz from these items you will have about 4400 health. I think that is far enough for a tank, and if it has enough armor and resistance, it will be indestructible. Especially Hecarim, who can be hardly to kill if he has a nice team.
Movement Speed, Attack Damage and etc. is useless for tanks.

This mastery has been build against players, not monsters or minions. Defense is simple, health, magic resistance, I think I dont have to explain. In utility, the Good Hand can be very useful at higher level, if you accidentally die. This can save your teammate's life, or your base, because as soon as you ressed, you can teleport to your team to rejoin to them. The Summoner's Insight is needed for Flash.

Usually I buy Doran's Shield first, thanks to its bonuses, mainly the health regen. You dont have to buy potions, cuz you can heal yourself with Spirit of Dread, and you have a good starting health regen in early games, so you can hold your lane quite long enough.
After you have farmed enough money, you can buy Mercury's Treads. It is very useful thanks to its passive, and the magic resistance bonus.

After this, you can start gathering some health. Buy Giant's Belt first, it gives you a lot of health, the complete with the other items. I think it is good to buy early, because you have enough time to stack it's bonuses.

Then Banshee's Veil is a nice item, it gives you health, mana (it is very important to have enough mana when charge into battle), magic resisance and the best, the spell block. It can help you to be more effective in the fight. There are so many nuking champion, and usually they try to kill you as fast as they can, so it can absorb the first damaging ability.

Next buy Randuin's Omen (if it is necesarry, you can sell Doran's Shield). After you charged into the group of enemies, activate Spirit of Dread, and use this item, so you can keep them near to you, and you will be nicely healed. Of course, the health, armor and health regen is always needed.

The next item is the Force of the Nature. I love this item, cuz it gives a huge amount of health regen, nice magic resistance, and thanks to its movement speed multiplier, you gain attack damage too. It is good to see that you dont have to be healed to see the little numbers jumping on your head.

And the last item is the Atma's Impaler. It is simply. More damage (cuz if you are alone, still you can nicely heal yourself against 2 or 3 enemy champion, and the armor.

In the early games, you dont have to be sacrifice yourself. You and your friend cant cause as much damage to heal you enough, because Hecarim is squishy in the beggining. Just simply run there sometimes, damage them with Rampage, or harass with Spirit of Dread, then back. If they attack, you can use Devastating Charge to flee, or to save your friend's life.

Devastating Charge can be also used to ambush enemies, or interrupt their abilities. Yes, you can interrupt every chanelling ability, for example Nunu's ulti or Fiddlesticks's Drain. Dont have to wait to deal much damage, use it proudly to interrupt those abilities.

Later, if you have enough health and resistance, and you gathered your teammates, you can start your ultimate tanking. First, charge into the middle of enemies with Onlaught Shadows (or if it is on cooldown, you can use Flash), IMMIDIATELY activate Spirit of Dread, the start spamming Rampage. The most important, to keep yourself as near as you can to your enemies, so you will be healed as your friends damage your enemies. 30% heal is not so bad, dont you think? If your friends deal 2000 damage in a short period of time, you will get 600 health, and that is very much for a tank. You can keep your enemies near to you with Randuim's Omen, or if you have a friend who can pull or toss enemies near to you.

If you have to flee, feel free to use your abilities or Flash. Dont forget, you can run trough units thanks to your passive.

Flash: As I said, it can easily save your life, or you can Flash into the battle to start tanking.

Teleport: If you die, or if you just simply want to buy some new items, after you have finished, you can easily rejoin to your team, and continue what you have started. Or you can teleport to you friend, and save his life.

Thanks for reading my build. Sorry if there are much grammar mistake, and sorry of it's poor design, this is my first build, and I dont know everything about it yet.
Feel free to leave a comment and vote, but please, as I said, notice this is MY opinion and build, and not the best or the only one option.