Hi, I'm Craig. I carry my home around with me everywhere I go, and this is mental_floss on YouTube. 
Today, I'm going to answer justineisftw's big question, "Why can't land turtles run or move fast? What makes them incapable of moving at a fast pace?"
First of all, Justine, thanks for making the distinction between land turtles and sea turtles for us. Sea turtles are actually pretty fast.
Leatherback turtles can reach speeds up to 22 miles per hour. Obviously, that's while swimming. I don't think they can scooch across the beach that quickly. I can't, and when I try, I get sand up my buttocks.
Anyway, land turtles are a whole different and much slower story. Let's get started. 

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For the sake of this video, we're going to classify land turtles as tortoises, as amphibious turtles and sea turtles have different anatomy, and move a little faster. Because Aesop wrote "The Tortoise and the Hare," and not, "The Slightly Less Slow Amphibious Turtle and the Hare. And/or Rabbit."
There are a few reasons why tortoises don't move very quickly. First is that they just don't have to. They're herbivores, which means they live off of vegetation, which is not exactly a fast-moving prey.
Tortoises are perfectly content to hang out in the field and lunch on grass; they don't have to chase after gazelle or catch salmon out of the rapids, so speed isn't exactly essential.
A tortoise's shell also has a lot to do with why they don't move very quickly. That shell is pretty heavy to lug around. The shell is also the tortoise's main protection from predators, so if something tries to attack them, they don't try to run away, they just curl up inside their shell, and it protects them. It didn't protect me in high school...
This seems like a fairly effective strategy, because no one wants to munch on what is mostly keratin, the same stuff your fingernails are made out of. No one... munches on fingernails...
Moving slowly also protects tortoises from injuring themselves. If you're not moving quickly, it's a lot less likely that you're going to break a bone or cut yourself. 
I use that same excuse when someone wants me to go on a run with them. "But if I move quickly, I might hurt myself!" Just kidding, no one asks me to go on a run with them...
So basically, turtles and tortoises are slow because they don't have to be fast to eat, their shell protects them from a lot of harm, and going fast means they have a greater chance of getting hurt. I wish I was a turtle... because I'm a lazy, lazy man.
If you have a question of your own that you'd like answered, leave it in the comments. Thanks for watching mental_floss on YouTube. See you next week... Bye...