Learn Russian: Мы решили поехать. Ехать километров двадцать. - We decided to go. The drive is about twenty kilometers.

This is how Russian expresses approximation! By putting the noun (километров) before the number (двадцать), it changes the meaning from exactly "twenty kilometers" to "about twenty kilometers."

Why is километров in the genitive plural form if it comes before the number?

The grammar rules for numbers still apply even when you invert the word order to show approximation. Since the number is двадцать (twenty), it requires the noun to be in the genitive plural (километров), regardless of which word is spoken first.

Why does the second sentence just start with the infinitive Ехать?

In Russian, you can use an infinitive to talk about an action itself as a concept. Here, Ехать translates roughly to "The drive..." or "To drive...". It is a very common, concise way to say "The driving distance is..." without needing extra verbs.

What is the difference between поехать and ехать in these two sentences?

Поехать is perfective and focuses on the start of the trip—they decided "to set off" or "to get going." Ехать is imperfective and describes the continuous process of the trip itself—the actual distance spent on the road.