Learn Polish: Czy byłeś wczoraj gotowy na ten długi spacer? - Were you ready for this long walk yesterday?

byłeś is the past tense form of być (to be). The -ł- part shows it is in the masculine past tense, and the -eś ending means 'you' (singular). So, we know the speaker is asking a male. If asking a female, it would be byłaś.

Why is the adjective gotowy used here instead of gotowa?

Adjectives have to match the gender of the person they describe. Because the verb byłeś tells us the person being asked is male, the adjective for 'ready' must also be the masculine form gotowy. If speaking to a woman, the sentence would be Czy byłaś wczoraj gotowa....

Doesn't the word na usually mean 'on'? Why is it translated as 'for'?

You are right, na usually means 'on' or 'to'. However, in Polish, the specific phrase 'ready for something' is always translated as gotowy na. It is followed by the Accusative case.

If na is followed by the Accusative case here, why does ten długi spacer look exactly like its basic dictionary form?

spacer (walk) is a masculine inanimate noun (a noun that is masculine but not a living person or animal). For masculine inanimate nouns, the Accusative case looks exactly the same as the basic Nominative form, so the words ten długi spacer do not change their endings.