Hecarim is a awesome jungler with high sustain, damage and mobility. Even when building tank hecarim can dish out a ton of damage with his low cooldown Rampage. Hecarim is also one of the best (My opinion) junglers in the game.

As for runes go i go with the traditional ADC rune set, which is working awesome. However you can put Movement speed Quintessentes for extra damage since your passive scales with your movement speed.

-Rampage- Maxing out Rampage as the first ability greatly increases your jungle clearing and gank success. When chasing try not to hit with auto attacks since it will slow your chase down, instead walk right beside your target and spam Q & W, only hit with a few auto attacks if you got some sort of slow like Red buff or Iceborn Gauntlet, or can last hit.

-Spirite of Dread- You would wanna start out with Spirit of Dread to give you some early life steal which is extremely helpful in early jungle. Max it out last as it is not your primary damage tool.

-Devastating Charge- Devastating Charge is your primary engage ability, be carefull not to activate it too soon otherwise you might not be able to reach your target. Using Devastating Charge from base with homeguard gives you enough speed to run to the outer mid turret still with homeguard on. When ganking with Devastating Charge try to make your knockback hit the enemies so that they go further back and not push them forward with it. Devastating Charge can also be used to quickly get away from most counter jungles, counter ganks and pretty much any solo champion, remember that Ghost can increase the damage and speed of your abilities.

-Onslaught of Shadows- Onslaught of Shadows can be used in varying ways. First of all it counts as your primary engage tool when teamfighting, ganking or catching enemies. When ganking allways save Onslaught of Shadows untill the enemy have used flash, engage with Devastating Charge to make them use their flash, and then as best as possible ensure that when you hit the enemy they are in the side of the landing zone (Circle of ult) that you want them to be feared to, making them run back into your team. Can also be used for splitting the enemy team in teamfights. Onslaught of Shadows can also be used as an escape mechanism over walls. Remember the fear will only hit thoose in the landing zone of your ult.

-Ghost- Using ghost increases the mobility and damage of hecarim. It is a great tool for both ganking and getting away in heated situations.
-Smite- Smite for Jungle ofcourse.

When playing as Hecarim you allways want to stay near a enemy for keep your Q on 2 stacks for maximum damage. When chasing down enemies do not use E and ram them from behind, run infront of them while spamming Q to try and knock them backwards. The fear on your ult can be specially usefull for diving towers, making the enemy team "Fear" out of the turrets range.

Clearing jungle with hecarim is not hard, focus the largest monster and let your Q & W kill the smaller monsters around. Allways start Hecarim jungle with blue since the high use of Rampage drains your mana alot.

As Hecarim jungle tank you should allways be the one engaging the fight if your team does not have a long ranged stun of some sort. Engaging in teamfights is allways done with Onslaught of Shadows if the enemy team is in a group. As a tank you will allways need to go to the backline and try to stop the enemy team's ADC from doing any serious damage. Do your best to scare off the damage dealers with your Q, use E to push away eventual threats that is after your own teams damage dealers.