Ok, so this is my first time as a GM and tbh I'm only just getting started into RPG's. So I'm going to be having a game very soon, I understand that this layout is meant to guide you through what to do as you play, However I have gone ahead and had a look through the first 4 encounters and the interlude to get a sense of what I'll need to do and how it will play out.

However, having a look at the interlude (after players have been awarded xp etc.) it mentions that PC's can choose to explore the valley if they wish, and to have a look at 'Other encounters in the valley' section.

This is where I get worried.

It is left mainly open as to what may happen (with exception of hunter encounters or talking to gatekeeper). Therefore my question is, should I have something prepared for these encounters, and if so, how in depth do they need to be?

(E.g. Lake has a ruin at the bottom, will they want to explore? The forest is tainted and at night requires PC's to take discipline check. How do you decide whether it's day or night? At the rockfall, will players be able to decide upon a knowledge check for the seated figure or do I guide them to such a conclusion?)

Thanks in advance

Well, how many RPG veterans are there? Are you all pretty much novices?

The Beginner Games are tutorials, so if beforehand you disclaim that this is a learning experience for everybody, you might not have to worry. That being said, if they do want to explore the valley and they go to a place, I just simply describe whatever it is the book says is there. Sometimes there's is something to find, sometimes not.

As this is a tutorial, telling them what skills they can use in what situations is a learning experience. Hopefully they will remember that in future games and come to their own conclusions. Counting time is different for everybody, but remember that every skill check takes up a period of time of varying length. Exceptional success may mean they did it quick, a multitude of threat may mean it takes much longer. Decide what time it is the game starts (mid morning, early afternoon, etc.), and figure how long each skill check might take; a Survival check across rough terrain might take half an hour to an hour, trying to sneak past bad guys might take a while, and so on. Ground travel distance from point a to point b might take X amount of time.

And if they do want to explore ruins, remind them that they are submerged. What equipment do they have? One pregen is amphibious, but do you know how cold a mountain lake can get in what appears to be winter time?

As Blackbird says, just explain you want to keep things "on the rails" while you learn the system. Still, the players will probably come up with solutions to challenges that the game doesn't prepare you for, so if you have to invent a challenge, remember that "even dice" means the PC will likely fail. If they are rolling 2 greens, and you say the difficulty is 2 purple, they will likely fail, or succeed with Threat. If you want the challenge to be "touch and go", make the difficulty one die less than the PC's pool.

(E.g. Lake has a ruin at the bottom, will they want to explore?

Athletics for swimming, and as Blackbird noted, probably a Resilience check because of the cold.

The forest is tainted and at night requires PC's to take discipline check. How do you decide whether it's day or night?

Personally I would decide what time of day they start, a friend who is a GM prefers to let the dice decide that. Do whatever you like best, then keep general track of how long the PCs have been going in game.

Thanks guys for the help, (we are all novices, myself having played some RPG's while the others have not)

Held first game and it seemed to go down well, although I must admit being a GM for first time can get stressful. Thankfully this first mission is laid out in such a way to help those first time role-players, and we all learned along the way. I shall definitely take into account that timing tip, as that will prove extremely useful, thanks.

I did miss some points along the way, especially at the last battle, but tbh I never expected everybody to want to continue after the interlude, however they were all keen to finish and enjoyed upgrading their characters with xp.

I look forward to playing Lure of the Lost and many further adventures, taking into mind my experience and your helpful tips and advice. Thanks!

Lure of the Lost has more freedom to it. It's more open for the players to explore, and there are certain consequences for not doing things or failing in a task. But good on you guys for having fun.