Learn Polish: Nawet jeśli masz dobrą wytrzymałość, ten maraton będzie trudny. - Even if you have good endurance, this marathon will be difficult.

Nawet jeśli is used with standard tenses (like the present tense masz) to talk about real or possible conditions. The speaker acknowledges it is quite possible the person has good endurance. If you used nawet gdyby, you would need the conditional mood (nawet gdybyś miał...), which would make it hypothetical or unlikely (Even if you were to have...).

Why is the adjective dobrą in the accusative case (ending in -ą), but the noun wytrzymałość doesn't end in -ę?

The verb mieć (to have) requires the accusative case for its direct object. Most feminine nouns end in -a and change to -ę, but wytrzymałość belongs to a group of feminine nouns ending in a consonant. For these words, the accusative form is exactly the same as the nominative. However, any adjectives attached to it still take the standard feminine accusative ending -ą.

Could I use the word kondycję instead of wytrzymałość?

Yes, dobra kondycja is very common in Polish to describe general physical fitness or stamina. Wytrzymałość is a bit more specific - it translates directly to endurance or toughness, emphasizing the ability to withstand prolonged, exhausting effort, which makes it a perfect fit for a marathon.

Why is it będzie trudny in the nominative case? Shouldn't we use the instrumental case after the verb (to be)?