I have quite a bit of trouble with trying to judge how much information to blast the reader with in 1 go, without it becoming boring, and without them losing track of it all.

Here's a section from the game / thriller thing I keep coming back to. The MC has not long started playing the game & I'm explaining the main quest that they need to achieve. Is it okay, or dull, or what?

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On the short walk to Woodtown, Alysia recalled the Elders explanation of her quest. The Elders were sending out several of their strongest apprentices to seek out the dragon orb, a jewel described in legends as being the colour of fire. The Elders believed that by acquiring this long lost artefact, its power could be wielded to gain advantage over the Drove. The orb was said to imbibe its holder with limitless mana. In the right hands, this would be an incredible force to unleash upon the enemy.
Alysia had been told to start her hunt for the orb on the island of Newbulus, where she would find curator Jalrek. The Elders believed that he had uncovered a document holding information as to the whereabouts of the mythical orb. She was to investigate this lead and follow it wherever it took her.
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Also, I know its a bit cheesy / cliche. But its supposed to be.

The first few pages before this are about a normal person in the present day starting up a computer game.

Then there's a bit more about this person entering the game (from their fantasy alter-ego's perspective) & getting off a boat & coming onto this island.

So what is the water of fiction? Action! It is the thing that gives life to plot and character, meaning to theme and imagery, and hope to the reader him/herself. Mix up the explanation with a little action.

Take the following passage:

"William recalled the words of the Elders: to find the lost Sword of Mightiness, he must go deep into the enchanted forest, find an old abandoned fortress, turn west, then follow the old river northward to where it emptied into Lake Witchidijji. Knowing his quest as well as the back of his hand, he set out at the borders of the forest."

This is a fairly interesting passage, but remember, you don't just want to give the reader information, you have to inform them and make them like it! The passage can be made more interesting:

William recalled the words of the Elders: to find the lost Sword of Mightiness, he must go deep into the enchanted forest and find an old, abandoned fortress. He looked up. There it rose like a dilapidated rhinoceros, high into the morning sky. The rocks were crumbling from time and water and moss. William wondered what ancient race of people had built such a monument. Then the voices of the Elders returned to him: he would now travel west to the great river."

The point is well-made: only give your audience information when they are ready for it. This particular chunk you've shown us is hard to judge (being out of context), but I would say that something pops to mind instantly:

"Alysia recalled the Elders [sic] explanation of her quest..."

This immediately made me think of an old episode of MST3K where a character walks into a room full of old records (which he then explains to himself) and Mike says, "Ah.. I see you've stumbled into the Department of Backstory..."

In other words, unless you're in a big section where Alysia's memories are important, this looks painfully artificial. You can tell that the author is "explaining," and it requires that the audience sit and listen carefully while the author tells them what is going on. Using memory instead of explicit "this is the quest" talk is a nice way of avoiding one of the infamous POV errors everyone on Hatrack seems to be obsessed with, but yes, it is too much. At least out of context.

And then you follow it with a long list of details that could almost be the outline of a power-point presentation. It explains, rather than presents what you are trying to get the reader to know. Sound familiar? Yes, "Show don't tell" is a pale, incomplete axiom. But it's a valid point.

The only way to explain a lot of information successfully is to keep the reader in the story while you are explaining. A good way of doing this is, as Ransom mentioned, to have something going on at the same time, but that isn't essential. Characters do have memory, and would think about their missions, but probably not in such explicit terms. People think in small flashes: images that stand out, lines of dialogue, and (most importantly)in opinion.

Thus, a good strategy in "recounting the mission" is to present the events as the character perceived them, rather than objectively. Tells us how the character is feeling about the events, how they reacted.

It follows, then, that something like:
"Alix remembered the day her father died. It was on a Tuesday in a hospital on Main Street, after he had overdosed on Abmbien and Schlitz beer, something he had done to avoid his shame after the market crashed. He had owned a lot of stock -- mostly in beverages and pharmaceuticals -- and when the crash came it wiped him out."
(Which is not terrible, but at least a little flat) becomes something like:
"Alix remembered the day her father died. It was in a room just like the one in Newton -- all white and smelling of alcohol -- but it wasn't as nice as the medicinal dive where the feds had dumped her. It was small and packed with weird, dramatic looking lights and machinery; one of the ER deathpits at that hospital on Main Street.
She put her head against the Isolation Room's Rubermaid protecto-door and started to cry. The a**hole had at least been supporting his own ruined stocks when he bought the Schlitz. She figured buying the Ambien had been a kind of joke -- it was a competitor's brand. He had gone into a little fit of nauseating self-pity, saying,‘That g*ddamn crash landed on me, Alex. Right on top of me. Like God threw a the NASDAQ at my head like a bolt of f**king lightning.' Then he swallowed the whole bottle of pills right in front of her. Like she wasn't even there."

It's longer, but I wouldn't call it diluted. It even has more information in it -- a lot more. But because it flows naturally from the mind of the viewpoint character, it doesn't seem like we're having a lot of information dumped on us at once. The language reflects the viewpoint character, and it feels more like we're just peaking into her head, which is what it should always feel like. Even when you're giving the backstory, it should still feel like we're just eavesdropping, not being told. Like Uncle Orson says, that's what they're really talking about when they say "show don't tell." It means, "Never make the reader feel like they're being told a story when you can make them feel like they're watching or living it."

So when you go into such explicit detail, almost bullet-point, giving us the specs on Alysia's mission, it sounds less like a story and more like a boring prologue that someone injected into the middle of a story. OF COURSE, this is just an opinion on something that is out of context. No one minds even internal prologuing if they're interested in what is being said. Hard SF (especially its sub-genres, like Cyberpunk)presents a few exceptions to the rules on obtrusive explanations. It's par for the course, in fact. But general fiction and fantasy almost never violate these rules. (Except in opening prologues, and really only Tolkein managed that one seamlessly. All the other attempts to make fantasy prologues are -- if you'll let me mix in some Zelazny -- just Shadows.) However, if you really feel like the reader would find it interesting to pause things and get all this backstory at once, then you've done nothing wrong. It is always possible to write in a way that gives information without stopping the story, though.

I'd love to see the whole thing; that would let me respond more accurately.