Learn Hardcore German: Die Fahrt in die Innenstadt dauert circa zwanzig Minuten. - The trip to the city center takes about twenty minutes.

Because in takes the accusative when it expresses motion into something, and the dative when it expresses location. Here it’s motion (you’re going into the city center), so accusative.

Motion: Ich fahre in die Innenstadt. (accusative, feminine: die)
Location: Ich bin in der Innenstadt. (dative, feminine: der)

What exactly does Fahrt mean, and how is it different from Reise or Strecke?

Fahrt: a ride/drive/trip by some vehicle (car, bus, train, etc.). Neutral about the means of transport unless specified. Very common for everyday trips.
Reise: a journey or trip that’s typically longer, planned, or more substantial (holidays, business trips).
Strecke: the route, stretch, or distance (the physical path between A and B), not the act of traveling it.

Why use dauert here? Could I say something like English “takes,” e.g., nimmt?

German uses:

dauern for durations: Die Fahrt dauert 20 Minuten.
brauchen “to need (time)” with a subject: Wir/Man braucht 20 Minuten (für die Fahrt / bis zur Innenstadt). Using nehmen for time (e.g., Die Fahrt nimmt 20 Minuten) sounds non-native; avoid it.

Can I start with an impersonal subject: Es dauert ...?

Yes:

Es dauert circa zwanzig Minuten, in die Innenstadt zu fahren.
Es dauert circa zwanzig Minuten bis zur Innenstadt. This is natural and common.

Why are there two instances of die: Die Fahrt in die Innenstadt?

They belong to different nouns in different roles:

Die (nominative) is the article for the subject Fahrt (feminine).
die (accusative) is the article for Innenstadt within the prepositional phrase after in (motion → accusative). For feminine nouns, nominative and accusative both look like die.

Could I say zur Innenstadt instead of in die Innenstadt?

You can, with a nuance:

in die Innenstadt (accusative): emphasizes going into the city center area.
zur Innenstadt = zu der Innenstadt (dative): “toward/to the city center.” Often used roughly the same way, but in die Innenstadt is more idiomatic when the idea is entering the center. Both are common in speech.

What’s the best English equivalent of Innenstadt? Is it “downtown”?

Roughly yes: “city center/downtown.” Other near-synonyms:

Stadtzentrum, Stadtmitte: neutral “city center.”
City: used in some places (especially larger cities) for the central shopping/business district. Nuance varies by region; Innenstadt is the standard term.

Is circa the same as zirka, and can I abbreviate it?

Yes. circa and zirka mean the same; zirka is the Germanized spelling. The common abbreviation is ca. You can also use synonyms like etwa, ungefähr, rund.

Where should I place circa? Could it come after the number?

The standard position is before the number: circa/zirka/etwa/rund zwanzig Minuten. You’ll hear zwanzig Minuten circa in speech, but in writing circa zwanzig Minuten or ca. 20 Minuten is preferred.

Should I write zwanzig or 20?

Both are acceptable. Style guides often recommend spelling out small numbers in running text (roughly one to twelve), but it’s not a strict rule. On signs, schedules, and concise info, digits are normal: ca. 20 Min.

Why is Minuten capitalized, and is the plural correct?

All German nouns are capitalized, so Minuten is capitalized. After numerals greater than one, use the plural: zwanzig Minuten. No article is needed with a bare quantity.

Can I add the means of transport? Where does it go?

Yes, with mit + dative:

Die Fahrt mit dem Bus/Auto/Zug in die Innenstadt dauert circa zwanzig Minuten. You can also front it: Mit dem Auto dauert die Fahrt in die Innenstadt circa zwanzig Minuten.

Is the verb in second position here? Can I change the word order?

Yes, German main clauses are verb-second:

Die Fahrt … dauert … You can front another element; the finite verb still stays second:
In die Innenstadt dauert die Fahrt circa zwanzig Minuten. In a subordinate clause, the verb goes to the end:
…, weil die Fahrt in die Innenstadt circa zwanzig Minuten dauert.

Can I omit circa? What changes?

Yes. Without it—Die Fahrt … dauert zwanzig Minuten—you present it as an exact or firm figure. With circa/etwa/ungefähr/rund, you signal approximation.

Is there a difference between Die Fahrt … and Eine Fahrt … here?

Die Fahrt … can refer to a specific, context-known trip (e.g., the one we regularly take) or to “the” typical trip in general.
Eine Fahrt … means “a/the typical ride,” focusing on one instance among many. Both are possible; Die Fahrt … often sounds a bit more definite or generic.

Could I phrase it with Fahrzeit or benötigen?

Yes, both are idiomatic:

Die Fahrzeit beträgt circa zwanzig Minuten.
Man/Wir benötigt circa zwanzig Minuten (in die/zur Innenstadt). These are slightly more formal/neutral variants.