Hecarim Jungle is fun, and I still do it. But being forced to get a ****ty jungle item seems to limit Hecarim's build efficiency to get the best stats possible. Also, Hecarim as a Champ is an insanely good split pusher, but being a jungler, you can't really bring TP. He isn't played on Top often though because his Q deals 66% damage to Minions, so his early farm is really bad. In the jungle it's bad enough but in lane it sucks even more. So if you're bad at CSing, you might not wanna try this build.
But Why Muramana? I don't think I saw a single Hecarim top build using this item. This item is popular with champs that either require a large amount of mana, or want to maximize their damage on single target spells. To me this fits Hecarim to a T. People like to make "One-shot-builds" for Hecarim based around dealing maximum damage with his E (A single target spell). Yet they never considered Muramana? Especially since the "Shock" passive procs on his basic attacks, which are empowered by his Trinity force? The damage output is insane. In case you don't know how Muramana works, you buy tear of the goddess and/or Manamune, and your spells (And basic attacks if you have Manamune) charge up the tear every time they are used, giving you more maximum mana. Usually you get 4 mana per use. Once you get 750 stacks on tear, you can buy your Manamune and Manamune transforms into Muramana, which significantly increases the item's capabilities.
The Passive "Awe" gives you AD based off of 2% of your Maximum mana. At 2500 Mana this is approx. 50 AD. the item's stat also gives you 20 AD, so you'd be getting 70 AD on this item. Not too shabby, you have a practically limitless supply of Mana and good AD. But the "Shock" passive is the crazy part. You spend 3% of your current mana to deal 6% of your current mana (Before use of the 3%) as extra damage on any Single Target spell or Basic attack. That can be about 130 extra damage if you're hovering near max mana. A little bit stronger than a Deadman's plate, and works consistently for every Auto and Single target spell. Basically this maximizes damage on Hecarim's E and Autos. However, combined with Trinity force, Hecarim can out DPS some ADC's, making him a terrifying sight for all the squishies on the enemy team. It's "Shock and Awe" in action when you one shot the ADC once and the enemy is forever scared of engaging you without the whole squad behind them, even if you're alone.

Trinity Force on Hecarim is a Given. WIth your q having such a short cooldown, the spellblade procs on your autos are insane with 200% (Or 3x) the damage. Not to mention the stats themselves are lovely.
Frozen Heart stacks well with Muramana, since you get 500 mana, not to mention you'll be capped off in CDR combined with your Trinity force (Since both give 20%). Also the 90 Armor and 15% reduced Attack speed for enemies around you makes you counter practically any ADC, which a good tank should try to do.
For situational Items, my favourite is Rightous Glory. I almost set it as a core item since I get it every match, but honestly it's all preference. You might want a ZZ'Rot portal or Deadman's plate for more consistent, passive speed rather than an AMAZING ACTIVE THAT GIVES YOU 75% MOVEMENT SPEED TOWARDS ENEMIES AND ENEMY TOWERS. But hey it's all preference. ALso it gives more mana, which means more damage for you, and the Eternity passive gives you some nice passive healing with your W and keeps your mana capped off at max for more damage from "Shock". Having only 500 health as a defense stat is the only downside, since there are items like Spirit Visage and Deadman's that give not only 500 health, but 50 armor or MR.
I don't like GA on Hecarim, but that's your choice. ZZ'Rot works better with him because you get the same stats as GA, but you get passive movementspeed around towers, and the portal is crazy strong for your already-dank splitpushes.

Normally you'd take Grasp of the Undying, but the thing is, in this build you don't have a lot of Health since you build damage, armor, and MR primarily. Since Hecarim's damage is based around empowered Auto attacks, the life steal on Warlord's is actually the most efficient for keeping his health high in battle. Strength of the Ages is also viable, but is not too great for in-lane because there aren't really THAT many cannons dying around you for you to get your stacks up. Also the 7% Penetration on Battering blows is lovely because in this build there isn't much room for you to get penetration, unless you got Maw of Malmortious or Youmuu's for some reason. Both not bad choices but not part of this build.

Armor Pen reds scale the hardest, and Hecarim scales REALLY well.
Health Yellows is my preference since you build plenty of armor anyway from frozen heart. That flat health early game is a bit more well rounded since there are quite a few toplane mages or people building Black Cleavers.
Movement speed quints are bae. Faster Pony is a happy pony. Also it gives you more damage on your passive, which is kinda like bringing some AD runes. Kinda.

Farming sucks. Start W because Q is worthless on Minions. Honestly you don't even need to use your q at all until you get Tear and a spellblade. It just ends up wasting your mana. Though feel free to waste mana when you get tear because stax yo. Your Autos deal more damage than your Q on minions so you'll be really sad when you try to last hit a cannon with a Q and miss it.

PROS:

Excellent DPS while maintaining your Tankiness:
FUll AD Hecarim, or going Full AD on any tank can be really fun, but their kit REQUIRES that they go up-close and personal, unlike an ADC with ranged autos and abilities. Being close means you'll take a lot of hits, so you'll probably die instantly if you have no tankiness. However, with this build, you'll have so much damage from your core items that you'll never feel lacking. **** that Full AD ****.
Probably one of the strongest toplane Splitpushers:
Those empowered auto attack from Trinity force smash towers, and what some people forget is that your E can also hit towers.

Excellent engages, good followups:
You're a FAST pony. If your jungler ganks you, you can get there for the follow up fast, and with a ton of burst.

SUPER strong mid game and Late game scaling:
CONS:

**** early Farm: Get gud

No harass whatsoever: Best you can do is slam into them with E and Q and try to make them back off.

Kinda Item dependent: Just like with Jungle, you want to emphasize farm to get your core items as fast as possible.