I tend to think most people who participate in endurance sports enjoy pain to a certain degree. If you train or race enough, you're bound to experience some pain at one point or another. After all, overcoming major obstacles while in pain (like a tough workout or a PR) is what tends to bring about endorphins and the oh so coveted "runners high". But I also feel like a lot of people run away from the painful part of training and racing, as if people gravitate toward long and easy all the time vs throwing down some tough intervals or tempo sets.
There is a saying that goes something like this - "Anyone can run long and slow, but not everyone can run fast".
This isn't to say that one is better than the other, such as saying going faster is better than going long and slow. They both have their places in training, depending on what you are training for and what you are trying to get out of training. It's just that a lot of people who have an interest in getting faster are afraid to run fast because it hurts and only a few are actually willing to admit that to others. These same people choose to just run in one gear all the time and see little progress and/or plateau, rather than train at different, faster paces depending on the intent of the workout. And that is what the quote means - anyone can go out there and glide along the trail effortlessly, but not everyone is willing to put in the effort to dig deeper to find pain and run fast.
This isn't to say that there aren't benefits to running easy or that people who run easy do so because they are afraid of run fast. What I am saying is that many people express that they want to get faster, but then resort to just running more easy miles in the hopes that they will magically get faster. These same people tend to be the ones that fear pain. And while there is some speed to be gained from "just" running, since you will still be building your aerobic system and thus, increasing your fitness, there are far more efficient ways to get faster...if that is what you are looking to do.
And when I use the term fast, I mean it as being relative to your abilities, not as a self-defined value of something like sub-5 minute miles. Fast running means pushing beyond your comfort zone (wherever that is for you), which for most people, their comfort zone tends to be somewhere between their long run pace and their other pace they sometimes run at. That other pace may be something resembling a "tempo" pace, but most people make their tempo pace either too fast for their abilities (typically based on a pie in the sky goal pace they don't have the fitness to train at) or too slow (because of said fear of pain). So what do you do about it?
I'm going to use one of my favorite sayings to summarize the key point - "In order to run faster, you have to run faster"
It's so simple, and yet so many people want to avoid it, like it is some kind of bad thing. They say things like "I'm just not build for speed". To me, that is just an excuse to opt out of even trying. Everyone can run fast, it just takes effort. So while the first quote I stated said "...not everyone can run fast", it is only in the context of those that are willing to face their fears. And this is why not everyone can run fast - those that don't aren't willing to face the fear of pain. Does going fast hurt? Of course it does! You may not feel like you can go fast, but I can assure you that you can. You just have to put forth the effort and try. It may hurt a bit more at first if it is new to you, but it will get (relatively) easier over time as you adapt to the feel of the effort.
Going fast is that time in your training when you stare at the little devil on your shoulder saying "slow down, it's much more fun that way" and flick him away because you are stronger than that. You battle the internal demons, but come out the victor. Not to mention, you begin recruiting additional muscle fibers (fast twitch and intermediate muscle fibers) than you would be if you were just training at your typical easy pace. This is where you develop the successful building blocks for being able to execute to your potential by maximizing the benefit of a given run.
I'm not trying to say everyone should run hard all the time, but what I am saying is that it should be part of your regular routine. Typically, once a week of shorter faster intervals and once a week of longer but still relatively fast tempo work is all you need. Everything in between is generally easy paced to allow you to recover from the intensity and stress from these faster workouts. Running faster teaches your muscles how to work efficiently while uncomfortable and your mind to shut up when all it wants to do is quit, both of which are skills that will serve you well on race day.
Without it, you can't push through the same mental barriers that may be holding you back from a breakthrough performance. Without it, you can't just magically go from running 9:00/mile in training to 8:00/mile in racing. And without it, you'll never toe the starting line of a race knowing that you trained to allow yourself to race to your potential and truly surprise even yourself on race day.