I think there's two things to bear in mind about Lady Audley's Secret:

It is a sensation novel, not a mystery novel. It certainly has a lot of elements of the modern mystery and detective genres, and (along with other sensation literature) did influence the development of those genres, but it predates them (in their modern forms at least). Therefore, while I understand why modern readers tend to approach the book with the expectation that it will conform to the standards of the mystery genre, I think we do the book and author a disservice when we do so.

Lady Audley has more than one "secret" throughout the book. I think there are at least three secrets, which I will refer to as: 1. The Initial Secret (bigamy; that Lucy Audley and Helen Talboys are the same person); 2. The Secret of the Cover Up (the conspiracy at Ventnor; pushing George down the well so her secret isn't revealed to Sir Michael); and 3. The Underlying Secret, the one which is referred to as "the secret of my life" and "the secret which is the key to my life" (that she's mad). Depending on your interpretation, there's also potentially another secret still (that she's not mad at all).

The Initial Secret, the bigamy, is the one I think most people are talking about when they say they guessed the secret. And yes, it is obvious. All the clues are fed to the reader within the first three chapters. Robert Audley doesn't enter the book until Chapter 4 and then spends a good chunk of the book investigating this secret (he also guesses the secret relatively early on, I think, and then sets about checking his suspicions).

The Secret of the Cover Up is also fairly obvious (the general gist of it, anyway, if not the details) relatively early. George's disappearance is what sparks Robert's investigation.

The Underlying Secret is perhaps less easy to guess, but not impossible. Robert doesn't guess this one ahead of its revelation, but the reader might.

So, yes, if a reader approaches the book with the expectation that there's one big secret and that it's going to take a lot of mental effort for the characters and reader to figure out, and that their suspicions will be confirmed or denied at the end of the book, they're likely going to be disappointed.

But I don't think that's what Braddon was trying to do.

Most of the dramatic tension of the novel does not come from the reader wondering "what is the secret". That's just one factor. Other factors include:

How information is revealed and concealed, both by characters and by the book itself

The escalation of events, secrets and stakes

The external conflict between the characters, especially the 'battle' between the protagonist and the antagonist (whichever way round you think they are - readers differ)

The internal conflict in the characters, especially Robert (e.g. duty to society vs duty to his family name; duty to his friend vs duty to his uncle)

Big Emotions

Questions of morality (e.g. where do different characters believe that a moral line is crossed, if at all? what do they think is forgivable behaviour? what do they think is justified-in-the-circumstances? what do they think is unforgivable? what do you, the reader, think?)

Lady Audley's Secret (and Sensation novels in general, from my experience so far) seems to be less about the 'destination' (i.e. what the secret is) and more about the journey, the drama and emotions along the way.