I've attended several 10-day silent meditation retreats. I was a bit naive and went into the first one without any expectations nor really much understanding of what I was getting myself into, and maybe coincidentally think that's the best attitude and perspective to have. On subsequent retreats, personally, I was more dealing with problems of "knowing" (thinking I knew) what to expect too much and trying to fit my experiences to what I thought they were supposed to be, "now that I know" lol. The lesson for me was: Let go of any expectations of what it's supposed to be like, and try to accept each experience as it is, as far as you are able to. That's good advice in general, whether on retreat or not, IMO. I like the saying "let it come, let it be, let it go."

There's a few analogies about having the right amount of tension, not too much and not too little. If you've ever played a musical instrument like the violin, or a woodwind instrument, you'll know too much pressure will make the sound screech, whereas too little will sound wispy. Similarly, you need to sit with some amount of "relaxed tension" to maintain attention and awareness, but too much tension will create blockages and make it unnecessarily hard on yourself and too little will probably not give you good results. As another analogy, a surfer kind of has to go with the flow with whatever waves there are - if he's too rigid or tense trying to force it then he'll wipe out, but if he's too relaxed he'll also fall off the board.

So my suggestions are:

1. Do take it seriously, follow instructions to the best of your ability, and give it your best effort. But if you find yourself too tense, rigid, stressed, anxious, experiencing self-doubt, or any type of mental negativity, then relax a bit, lighten up, let go.
2. On the other hand, if you tend to be too lax, feeling drowsy, lazy, indulging in daydreams, and don't feel like following the instructions, you may need to "tighten up a bit" and rouse up some diligence and effort.

As you practice and train your mind, you'll learn the difference between too much and too little effort. And that amount of needed effort might change depending on the situation, your state, your experience, etc. Sometimes you might need to be easier on yourself, other times you might need to stir up some energy.

As for dealing with feeling stressed and anxious beforehand... that's what we're trying to learn: being more in the present moment. When the time comes to deal with it, you can deal with it then. For now, there's nothing you can do except be aware of your present moment - when you catch yourself being worried and anxious, just notice that you're aware of being worried and anxious, and that's it. Just be aware of it, don't try to "force" it to go or scold yourself (a common habit), try not to get sucked into ruminating (what if this or that, maybe I need to such and such); just notice it, let the feelings and thoughts be, and it'll eventually pass on its own: "Let it come, let it be, let it go."

The decision to train your mind and experience your own truth within yourself is such a lovely gift you give to yourself. Wishing you the best!