Darksiders

Found a way that took around 1 hour 20 minutes. Not sure how many miles i already had but imagine it was only around 20.

Anyway, if you go to the soul bridge, and pull down on the left stick, when you reach the end you will do a sharp stop and 180' turn on the spot. So long as you have it right you will go back on the bridge. Leave this running and with the increased speed from the bridge you will be done in no time.

Leave a rubber band or something keeping the stick held down and you dont even need to hold the pad.

Dark Rider -
Ride for 100 miles

If you start running on Ruin, then press RB(for the Ruin's Charge) and all use all 5 of them once the previous fire effect from charge runs out .
20 second from the time you start running till all the Ruins charge abilities(x5) are back, this will equal around 0.33 of a mile.
Should be able to do approximately a mile a minute,
Approx 1hrs 40mins to get achievement.

The other way is move to the centre of the arena, press the thumb stick back and to the left and then weight it down in place (this will make Ruin run circles, you want to be in the middle so that you dont run over the fire traps and end up dieing.
In about 45 minutes you will gain approximately 32miles
Approx 2hrs 40mins to get achievement

Another way, is the arena where you get Ruin, there is a locked door on the north end, plant your horse to the left of this door and push forward on the stick, even though the horse is cantering on the spot you can check the distance travelled and it is still going up.
Just find something that puts a little weight on the stick to keep it pushing forward, and go and do something for a few hrs you'll get this achievement while being productive on something else.
Only use this way if you have about a day to spare as you will only get around 5miles in about 45mins

Times based on ingame calculations

Personally, I found the best place to do this was in the canyon at the edge of the Ashlands leading to the Iron Canopy - ya know, the area right after the Soul Bridge. Sure, you could just run Ruin around in the Stygian arena forever, but the scenery sucks, and it makes a bland, grinding Achievement even more mundane. The canyon is a long run (approx. .7 miles from the Ashlands end of the bridge to edge of the cliff), devoid of enemies (except for a randomly appearing horseman, who is easy to avoid), and the Soul Bridge offers about .18 miles per lap of full charge speed without burning a boost. As an added bonus, every time you return to the Soul Bridge the game Auto-Saves your progress, assuring you no lost mileage or time! Sweet!
Simply charge across the bridge, run the length of the canyon - boosting as you go - then swing around at the edge of the Iron Canopy cliff/entrance, and race your way back and across the bridge again. I've clocked it at just about 45 seconds per length, or about 1 minute 30 seconds per lap, logging you 1.4 - 1.5 miles per lap.
Yeah, it still sucks, and it is a grind, but I found this method far less mundane than simply running in circles, and it might actually be a little faster, given the free boost speed across the Soul Bridge. :)
Hope this helps! PROST!

This one is called Dark Rider, you have to ride on horseback for 100 miles. This one is impossible to get without grinding, you'll get a few miles when you first get Ruin. But after that you will have to grind for it.

The best way of doing this is go to either the arena where you first get Ruin or the area you fight the Stygian and hold the left stick to the side to run in circles.

You can also run around the Ashlands and a couple other places to grind for kills on horseback, which is another secret achievement

The easier way to do this instead of getting sore eyes for hours.

Go to the Stygian's Arena with your ruin, and simply find a small object you can place between the D-pad and the left thumb stick so that Ruin will just run around aimlessly in circles.

I used a pick from playing guitar that worked nicely or you could possibly cut out a convenient shape from cardboard to place nicely in between.

Simply go out for the day, kick a ball with a friend, then come back whenever you feel to embrace your lovely cheeve :)

Between the soul bridge and the iron canopy, simply stand your horse in the middle-ish of the area, hold down-left (or right) with your left thumbstick and put an elastic around the thumbstick and the left "handle" of the controller. Make sure your horse is going nicely in circles without hitting anything, and then go and do something interesting for a couple hours.

If you want to get this achievement before you finish the game, (assuming you arent beating the game in a single sitting...) when you stop playing, or take a break or anything, always set your horse up to do circles. This way, with 5-15 minute spurts of circles, you can slowly get the achievement without having to lock up your precious xbox for hours on end. This allows for more achievements to be unlocked, while passively grinding this one. Wow! incredible!

Like the other solutions have said, go somewhere with some open space and no baddies (I did mine near the entrance to the ashlands).

Use a rubber band wrapped around the analog sticks to pull them towards each other (causing war & ruin to run in a circle), plug in your controller and then go do something else. I got the achievement in under 2.5 hours.

To add to the (multiple) other solutions suggesting you ride your horse Ruin on the Soul Bridge (in Ashlands, right before Iron Canopy).

It takes approx. 12 "runs" across the Soul Bridge to clock up 1 mile (using in-game "Distance Traveled"). This takes approx. 1 min 20 seconds to complete 12 "runs."

Assuming you have 0 miles run (which is impossible lol), it would take approx. 1,200 "runs."
This takes approx. 130 minutes (A little over 2 hours). That is the MAXIMUM time.

More likely, you've already clocked up approx. 20 miles, which would have about 960 "runs" to complete 100 miles.
This 80 miles on the Soul bridge would take approx. 105 minutes (about 1 hr 45 minutes).

Just to continue the math.. let's do 50 miles, exactly 50%.
That's 600 "runs" taking approx. 65 mins (A little over 1 hour).

Immediately after obtaining the below achievement, defeat the enemies while riding Ruin using attacks:


Position War, riding Ruin, close to the center of the Arena. The placing need not be exact. Take note of the distance traveled by pressing and going to the stats tab. Take a short rubber band (or a longer one doubled up) and wrap it around the and the left handle of the controller so that is depressed in the southwest direction. This will cause Ruin to run around in circles in a counter-clockwise direction.

Take a break. If you want to check the progress, press and look at the distance traveled. It took me about two and a half hours to travel the required 100 miles and unlock the achievement.