Der Weg zum Gipfel ist lang, aber unsere Ausdauer reicht aus, um oben anzukommen.

Zum is simply the contracted form of zu dem.

• zu requires the dative case.
• Gipfel is masculine: der Gipfel in the nominative.
• Dative masculine of der is dem, so: zu dem Gipfel.
• In normal spoken and written German, zu dem usually contracts to zum.

So:

• zu dem Gipfel → zum Gipfel (both are grammatically correct; zum is more natural here).

Gipfel here is dative singular.

Reasoning:

1. The preposition zu always takes the dative.
2. The article der Gipfel (nominative) becomes dem Gipfel in the dative.
3. zu dem Gipfel contracts to zum Gipfel.

So if you see zu (or zum/zur), expect the following noun to be in the dative.

Unsere Ausdauer is in the nominative, because it is the subject of the verb reicht aus.

• Wer oder was reicht aus? → unsere Ausdauer.
  (Who/what is sufficient? Our stamina.)

Ausdauer is a feminine noun:

• Nominative feminine: die Ausdauer
• With the possessive unser- in the nominative feminine: unsere Ausdauer

Unserer Ausdauer would be dative or genitive, which we do not need here, because nothing in the sentence is demanding dative/genitive for this noun. It is simply the subject.

Reicht aus comes from the separable verb ausreichen, which means to be enough / to be sufficient.

• unsere Ausdauer reicht aus = our stamina is enough / is sufficient

You could say:

• Unsere Ausdauer ist genug – understood, but sounds a bit off / less idiomatic.
• More natural: Unsere Ausdauer ist groß genug (our stamina is big enough).
• Or: Unsere Ausdauer reicht (everyday, you can omit aus in many contexts).

ausreichen is standard and idiomatic when you want to express sufficiency:

• Das Geld reicht aus. – The money is sufficient.
• Die Zeit reicht nicht aus. – The time is not enough.

Because lang here is a predicative adjective, used after the verb sein (to be).

In German:

• Attributive adjectives (placed directly before a noun) take endings:
  • der lange Weg – the long path
• Predicative adjectives (after sein, werden, bleiben, etc.) do not:
  • Der Weg ist lang.
  • Der Gipfel ist hoch.

So lang stays in the base form because it is linked to the subject by ist, not directly modifying a noun before it.

um oben anzukommen is an um ... zu + infinitive construction. It expresses purpose – roughly, in order to in English.

Structure:

• um
  • [rest of clause] + zu
    • infinitive

In the sentence:

• um – introduces the purpose
• oben – adverb (where? at the top)
• anzukommen – infinitive of ankommen with zu (to arrive)

Meaning: in order to arrive at the top / in order to get to the top.

More examples:

• Ich lerne Deutsch, um in Deutschland zu studieren.
• Er spart Geld, um ein Auto zu kaufen.

Because ankommen is a separable verb: ankommen = an- + kommen.

Rules:

• In the infinitive with zu, zu goes between the prefix and the verb stem:
  • ankommen → anzukommen
  • aufstehen → aufzustehen
  • mitkommen → mitzukommen

You never write zu ankommen or zu aufstehen.

So:

• oben anzukommen = to arrive up there / to reach the top

Oben is an adverb meaning up / up there / at the top. It already contains the idea of location, so you do not need nach here.

Nuances:

• oben anzukommen – to arrive up there / at the top (general, less explicit)
• auf dem Gipfel anzukommen – to arrive on the summit (more concrete, names the place)
• nach oben kommen – to go upwards, more about the movement upward than the final place

In this sentence, oben anzukommen matches the metaphorical tone: the top as a goal, not just the geographical summit.

Aber here is a coordinating conjunction. It does not take the verb to position 2 like English but does or like some other German structures.

In a main clause with aber, normal word order continues:

• Der Weg zum Gipfel ist lang, aber unsere Ausdauer reicht aus.
  • Clause 1: Der Weg zum Gipfel ist lang.
  • Clause 2: unsere Ausdauer (subject) + reicht aus (verb).

You would only move the verb in front of the subject if you deliberately changed the structure for emphasis, e.g. in a question-like pattern, which would sound marked or wrong here.

So: aber + [normal main clause word order].

Yes, you can say Der Weg auf den Gipfel ist lang. Both are grammatical, but there is a nuance:

• Der Weg zum Gipfel – more neutral; zu
  • dative often used for directions towards a point/goal.
• Der Weg auf den Gipfel – emphasizes going onto the summit (using auf
  • accusative to show movement onto a surface or height).

Both would normally be understood as “the way to the summit is long,” but:

• zum Gipfel = towards that place (the summit as a goal)
• auf den Gipfel = up onto that high point (more physical “climbing onto” feel)