Exercise will build muscle mass and strength, but what you want is increased speed and efficiency. To achieve that requires that that 
you get off the exercise bench and convert static routines into action (running comes to mind). Grace and speed are not the same as muscle mass and strength. Think of ballet. It requires strength to be sure, but the goal is to display smoothness and grace in movement.
Same for writing. Muscle, i.e., things like knowledge of grammar, will help you hone your basic writing skills, even endurance, but the truly desired results come from the grace and flow of your prose. As in ballet, that requires focus and control and practice.
I recently ran across an online article that listed “6 Exercises to Improve Your Writing Skills.” That’s the sort of title that grabs the attention of virtually anybody who puts pen to paper. Our ilk is in constant search of the magic formula of instant perfection! But, I have to say that I’m no longer sure just what the term “writing skills” actually entails. My personal goal is to improve the quality of my writing with each effort. Improvement comes incrementally, and, hopefully, with each effort I try to add an increment.
The underlying skill set includes a healthy vocabulary, an understanding and reasonably firm grasp of grammar, an understanding of just what a paragraph is supposed to do, a sense of sentence structure and organization, and having some good ideas to write about, and, ultimately, an ability to take a mundane topic and make it sing a little bit. Or you can visualize a smithy taking a piece of red-hot steel and forging it into a horseshoe. You have to pound the hell out of something to create a useful result!
Here’s the list of suggested exercises to improve your writing skills from the above-mentioned article I recently ran across:
• Pretend you’re somebody else.
• Turn the news into a fictional story.
• Write an obituary or eulogy.
• Rewrite a fairy tale or other classic story.
These are tried and true exercises that can open creative doors for young students, and I used some of them when I taught in the classroom. Additional suggestions include using reference books. The two short paragraphs that followed that suggestion had me scratching my head about just what the writer of the article actually meant. I quote the two graphs in case you might be better qualified to figure them out:
“It might be odd to go to an encyclopedia or telephone directory when you’re stuck on a creative writing project. But these resources 
can give you inspiration. Use the phone book to come up with character and street names.
“Flip through an encyclopedia casually and you’ll be surprised at how the entries may help you. Find your character a hobby or home country. Create a backstory through a historic or political event. See what else you come up with.”
Spare me. The suggestions even sound like busy work. In fact, they sound like a lazy teacher coming up with assignments with little thought invested in their value. Want some inspiration? Go into a public restroom and jot down the scribbles and graffiti you find on the wall.
Or you could just piss up a rope. That’s essentially what such exercises have you do. It should be a felony to pass off busywork, if not useless exercises, as substantive help.
If you want to write, write. If you want to get better at writing, write a lot, and if there isn’t someone handy to provide you with feedback, set your efforts aside and work on something else for a while. Come back to it in a few days, a week would be best, and see how it flows and whether it says anything and says it well. Analyze you work as if someone else wrote it. (That’s why you wait the week. Time provides a little psychic distance!) When you find shortfalls, and you will, identify and describe them in your own words, then fix them. Rewrite the piece. Several times, if needed. If you find nothing to fix or make better, you’re either a reincarnated Hemingway or have a much too high opinion of the quality of your work.
You might find your best response to your writing efforts lies in the direction of the wastebasket. If so, start over. The wastebasket 
won’t be offended. It’s what it does. The bottom line is: if you have something you want to write about, write about it. So what if it takes a couple dozen tries to get it to the point where you begin to feel pleased. The goal is to knock your own socks off. If you can do that, you’ll knock the socks off others. And, rather than getting embroiled with exercises, you have instead focused on your primary goal—to improve and moved your project forward. Why would you want to spend time writing exercises to get better rather than working on writing what you want to write better? Focus on the stuff you want to write.
There are zillions of articles out there—and obviously this piece adds another one—on how to write or how to write better or perform some aspect of story telling more effectively. Don’t get me wrong. I’m not condemning such efforts; I’m guilty of generated many of them. The writings of others in and of themselves can serve as manuals on writing—read and analyze them for how good or not you think they really are. Edit them. Rewrite them. Be as objective as possible in coming up with ways to either improve them or to create an alternative approach. Always ask yourself how would you have written this or that? Your job is not to copy the efforts of others, but learn from them—techniques, word usage, organization, structure and so on. Learn to apply the concepts, not copy the examples. And more importantly, work on learning how to objectively self-critique. These are the activities that are part and parcel of the writing process. You want to get good at each one of them.
Just because somebody else has written something doesn’t give it its credibility. It’s the impact a piece has on the reader and how 
readers react to it. You want to analyze why you react the way you do to a piece to determine what gives it credibility, or the lack of it. Most of us innately know when a piece of writing is just okay and when it’s really good, even our own. But we need to take the time to determine specifically why one piece is mediocre and another lights up your soul. Only when you determine why something is really good can you develop and apply the standards and techniques to your own efforts. Be warned. This is an ongoing, never-ending process. Keep it up and you’ll find yourself rewriting the ad copy for television commercials!
Reading a few paragraphs on how to write better and then trying to apply the suggestions demands thought. Writing is not a physical process; it is a mental one. It can be arduous. Some surgeons fix appendices about to burst. Some remove brain tumors. Writing is more like the latter. If it weren’t we all would become terribly good at it.
So here’s the secret. To become a better writer you have to write, but write substantively—with focus and intent and purpose. It’s like learning to dance ballet. And remember, a prospector may get rich from gold, but he had to first learn where and how to find it. Where a prospector might trip over a large nugget, a writer has to create her own.