Путь к морю был долгим, но стоил того.

Because the preposition к normally requires the dative case.

• море = nominative singular
• морю = dative singular

So:

• путь к морю = the way/path to the sea

This is a very common pattern in Russian:

• к дому = to the house
• к другу = to a friend
• к городу = to the city

So к + dative is the key rule here.

Долгим is the instrumental singular masculine/neuter form of долгий.

Here it is used as part of the predicate after был:

• Путь был долгим = The journey/path was long

In Russian, after forms of быть (to be), an adjective describing what something was is often put in the instrumental case, especially in standard written Russian.

So:

• долгий путь = a long path/journey
  
  • adjective directly modifies the noun
• путь был долгим = the path/journey was long
  
  • predicate adjective, often instrumental

You may sometimes also see nominative in some contexts, but был долгим is very natural and standard here.

Был is the past tense masculine singular form of быть (to be).

It agrees with путь, which is a masculine singular noun.

Compare:

• путь был долгим = the path/journey was long
• дорога была долгой = the road/journey was long
• путешествие было долгим = the trip was long

So the form of быть changes depending on the gender and number of the subject in the past tense.

Стоил is the past tense masculine singular form of стоить.

Here it agrees with the same subject: путь.

So the second clause really means:

• но [путь] стоил того = but [the journey] was worth it

Russian often leaves out the repeated subject if it is already clear from context. English can do this too: The trip was long, but was worth it—though English usually prefers to repeat it more explicitly.

Стоить того is a very common Russian expression meaning:

• to be worth it
• to be worth the trouble / effort / wait

Literally, стоить often means to cost, but in this construction it means to be worth.

Examples:

• Это стоит того. = It is worth it.
• Поездка стоила того. = The trip was worth it.
• Ожидание не стоило того. = The wait was not worth it.

So in your sentence, но стоил того means but it was worth it.

Because стоить in the sense of to be worth normally takes the genitive case.

The pronoun то changes as follows:

• nominative: то
• genitive: того

So:

• стоить того = to be worth that / worth it

This is a fixed and very common expression. You should learn стоить + genitive as a pattern:

• стоить времени = to be worth the time
• стоить денег = to be worth the money
• стоить усилий = to be worth the effort
• стоить того = to be worth it

Because но means but, and in Russian it normally connects two clauses here:

• Путь к морю был долгим
• но стоил того

Just like in English, a comma is typically used before but when joining two clauses.

So the punctuation is completely standard.

Путь can mean several related things:

• path
• way
• route
• journey

In this sentence, it is not just a physical narrow path on the ground. It can refer more broadly to the way to the sea or the journey to the sea.

That is why a natural English translation often uses journey or trip, even though the Russian word is literally closer to path/way.

Yes, it could, but the nuance changes a little.

• путь often sounds broader, more abstract, or more literary: way / route / journey
• дорога more often suggests a road or the trip itself in a concrete sense

So:

• Путь к морю был долгим sounds a bit more elevated or narrative.
• Дорога к морю была долгой is also natural and may sound a bit more concrete.

Both are possible, but путь is a very good choice in this sentence.

Russian often omits words that are easy to understand from context.

The full version would be:

• Путь к морю был долгим, но путь стоил того.

But repeating путь would sound unnecessary. So Russian naturally leaves it out:

• Путь к морю был долгим, но стоил того.

English does something similar sometimes, but Russian allows this kind of omission very comfortably when the subject is obvious.

It is natural standard Russian, but it has a slightly literary or elegant feel, especially because of:

• путь instead of a more everyday word like дорога
• the expression стоил того
• the instrumental predicate долгим

It is not old-fashioned or unnatural at all—it just sounds a bit more polished than very casual speech.