So I can see what you did there, starting in media res to put down a strong foot at the start. But as everyone else has said: too fast. Very little description, too speedy, you've heard that already. While the chase in the woods is meant to be vague, as to reflect how this Isaac character himself is lost in the action, that doesn't mean he's completely lost. Can he see? What does he see? Can he hear? What does he hear?
Now, the thing with starting in media res is choosing how to progress back to the past from that. In the midst of this dark figure, does Isaac decide to hit the pause button like Zach Morris? Maybe his life is flashing before his eyes, like it might happen in near-death experiences. It's hard to comprehend how Isaac can tell us his whole story in this little moment wherein he might have three seconds before he dies, or at least gets hurt, surprised, whatever you were planning.
One thing Miss Bay up there pointed out was the jump from first person to third person. In first person point of view, where the narrator is a character in the story and uses I, me, us, and so on, the knowledge of the whole world is limited, and the story has to follow him/her. In third person, wherein the narrator is simply a narrator and may not be a character (but may, too, like Bob Saget in How I met Your Mother), they know all, see all, but limit emotional impacts like catharsis and development; it's still possible, but that's ultimately up to the author. Basically, you have to choose one and stay with it for the rest of the story. Of course, the point of view may change from one character to another, and even from fist to third, but that's rarely pulled off. I've only seen it done in a visual novel series known as Harvest December, and even then I got lost on occasion with regards to it.
Here's a term for you: "stream of consciousness". This stream is a river of sorts; the events of the story have to flow as smoothly as a river. Not necessarily linearly, going from the middle to the beginning is possible, and has been done countless times (The Hangover movies usually did so), but you have to bridge them properly. Like I said regarding the dark figure, I would either go with life-flashing-before-his-eyes or Isaac hiding from the figure as he recalls everything; something to bridge everything.
Pacing and description is everything. While you know everything of your work, we the readers do not. Tell us everything. Where do the characters live? Is Isaac barely starting as a proper Trainer? Has he been one for a while? Who's Steven? Steven Stone? Steven Universe? Steven the dentally challenged? Was Surskit a gift, or caught? And why on earth did he ask it to "sticky web" his room?! I feel the undying need to find an answer to that last one. And ultimately, does he have a purpose, a journey, anything to get his butt out of bed in the morning for? Or is this a story that just happened to a casual guy?
And now, grammar. Grammar, grammar, grammar. You need to master proper grammar. A saying I grew up with states: "You need to hone your skills, practice hard, and master the craft you want to perfect. Only then, are you allowed to break the rules." This applies to almost any craft, but especially writing. While people online can read grammar mistakes, abbreviations, emojis, and any kind of 1337 speak, it's still not suitable for grammar unless justified (Super Paper Mario, anyone? XP). A reader looks for readable material. I, in my old age of 22, prefer to see the whole word instead of the abbreviation; I wanna see "by the way" instead of "BTW" unless the character actually says the letters B, T, and W. I know this is just a fanfic, but you should treat it as if it were a novel, in the case that you write a work that is completely original, not a fanfic, and some publisher wants to publish it and make you fat stacks of mad money. But nobody would do so if they can't understand what you write. Of course, there's also the thought of caps, bolding/italicizing, and underlining anything; I'm guilty of doing some of those in my little work in this sub - especially capslock - but should enough people say it's unpleasant, I'll willingly stop (so far, I've gotten only one complaint, and they're for the most part tolerable of my use of it; I hope it's okay for everyone else).
Now here are my tips and notes for you. A step-by-step guide to writing most things, as I've done so from pen to paper long before I submitted a thread here. I still feel my tricks work in any writing I do, and I hope they work for you. So, let's begin:
1. Write an outline.
An outline is a sheet of paper or document that literally outlines the points of the story. Think of it as a timeline of the book, summarized and stripped down to a skeleton. List all your chapters in bullet-points or dashes, whatever works for you, and then name the chapter as something that grasps the main idea or event presented in it (not really name chapters; just name them for your sake, to keep you from forgetting what it's about). Under them, list important events that will occur. Then, with all in sight, write it out. Voila. This works for every chapter.
2. Think in an episodic paradigm.
Let's look at Breaking Bad and the equally-awesome Better Call Saul. They work as long-running shows, yes, but they have, well, episodes. Think of every chapter as an episode. While cliffhangers are nice here and there, nobody likes that in a constant frequency. So, allow me to tell you right now: do not post any chapter unfinished. Don't come onto this thread and just write from scratch. That's far too much; for the reader to stomach, and for you, the author, as there will be anxiety when left with a blank canvas. Do what I do: open up Word, write out your chapters, and save them for later. Only after you've completed one, edited it, and refined it, should you be allowed to post it. Have I fallen through finishing a chapter? Yeah, but I learned. XD So think of all chapters as episodes, as short stories that come together to form a larger story, if you will. This works in The Hunger Games books. Each has three parts, with nine chapters in each, leading to a total of twenty-seven chapters per book, for a grand total of 81 chapters in the series. Each is episodic. Even the parts themselves are:
Book 1 Part 1: Introduce the characters, Panem, and the games.
Book 1 Part 2: The games themselves.
Book 1 Part 3: Endgame and victory.
It works. And as a last note on this subject, there's a difference between a cliffhanger and leaving an audience with mouths open begging for more: it's called suspense. If you can work with it, then there's no need to post sneak-peaks at future chapters.
3. Keep notes.
You know everything of the story, but we don't; that's to your advantage. Keep notes on all characters, places, etc. to look at later. Does Isaac have damning character flaws? You can totally throw in easter eggs that hint at them! Implications can make for an enjoyable experience! To this day, I can't tell what kind of hell the Diamond Authority themselves can personally cause in Steven Universe, and such a mystery is one of many that keeps me hooked. On top of whether or not Rose Quartz is still conscious or completely dead. But I digress. You can totally get away with making mental notes instead. I do, but that's because my work here has few characters of importance, so it's easier to track everything. If you have more than mine, then I highly implore you to make a physical document.
4. Think ahead before you write ahead.
Don't just write a story that begins with a boy and his Surskit. Make sure you know everything: how it begins, how it flows, and how it will end. If you can see all of that, and it turns out that you hate how it flows, then you can edit that thought long before you've already written it and made it too late to change things. Trust me, you'll like that kind of creative power, and be thankful for having it, so you won't make mistakes. You don't want to constantly go back and edit posts, do you?
