फ़िटनेस क्लास न तो लंबी है, न ही मुश्किल है।

It is a common Hindi pattern meaning neither ... nor ....

So:

• न तो लंबी है = it is not long
• न ही मुश्किल है = nor is it difficult

Together, the sentence says that both qualities are being denied.

This is mostly an idiomatic paired structure.

• न तो introduces the first negative item
• न ही introduces the second negative item

You should learn न तो ... न ही ... as a set expression. It is one of the standard ways to say neither ... nor ... in Hindi.

The individual words do have other uses in Hindi, but in this pattern, it is best to treat them as part of a fixed construction.

Because क्लास is treated as a feminine singular noun in Hindi, and लंबा is a variable adjective that agrees with the noun.

Agreement pattern:

• masculine singular: लंबा
• feminine singular: लंबी
• masculine plural: लंबे
• feminine plural: लंबी

Since क्लास is feminine singular, the correct form is लंबी.

Because मुश्किल is an indeclinable adjective in this use. That means it usually does not change form for gender or number.

So you can say:

• किताब मुश्किल है = the book is difficult
• काम मुश्किल है = the work is difficult
• क्लास मुश्किल है = the class is difficult

Unlike लंबा, which changes form, मुश्किल stays the same.

Yes. Loanwords in Hindi still get assigned a grammatical gender, and क्लास is commonly treated as feminine.

That is why you get:

• क्लास लंबी है
• not क्लास लंबा है

This is something learners often just have to memorize, because grammatical gender in Hindi is not always predictable from English.

Because each half of the neither ... nor ... structure is basically its own clause:

• न तो लंबी है
• न ही मुश्किल है

Hindi often repeats है in this kind of coordinated sentence. It sounds complete and natural.

In very casual speech, people sometimes shorten things, but in standard written Hindi, repeating है here is normal.

Hindi does not use articles the way English does.

So Hindi usually says simply:

• फ़िटनेस क्लास

and the context tells you whether English would say the fitness class, a fitness class, or just fitness class.

This is very normal in Hindi.

Yes, that is also possible.

For example:

• फ़िटनेस क्लास न लंबी है, n मुश्किल है।

This is shorter and still understandable. But न तो ... न ही ... sounds a bit more complete, balanced, and polished. It often feels slightly more formal or emphatic.

So:

• न तो ... न ही ... = very standard and nicely structured
• न ... न ... = simpler and also common

This is a normal Hindi predicate adjective structure.

The pattern is:

• subject + adjective + है

So:

• क्लास लंबी है = the class is long
• क्लास मुश्किल है = the class is difficult

Hindi does not need a separate word like English is before the adjective. Instead, the adjective comes first, and है comes at the end of that clause.

Yes, both spellings can be seen.

• फ़ uses a nukta and is meant to represent an f sound more accurately
• फ is the older basic Hindi letter, and many people still use it for borrowed words

So both फ़िटनेस and फिटनेस may appear in real life.
The version with फ़ is often considered a more precise spelling for English-origin words with f.

A rough pronunciation is:

fitness klaas na to lambee hai, na hee mushkil hai

A few notes:

• लंबी sounds like lam-bee
• मुश्किल sounds roughly like mush-kil
• ही is a long hee
• क्लास is pronounced close to class, but with Hindi vowel length

This is only an approximation in English spelling, but it is enough to help you get started.