Learn Spanish (Latin America): ¡Qué colina tan empinada! Incluso el sillín me duele ahora. - What a steep hill! Even the saddle hurts me now.

Yes! Both incluso and hasta mean "even" and are completely interchangeable in this context. You could perfectly say Hasta el sillín me duele ahora. Incluso can sound slightly more formal in some regions, but both are very common in everyday speech to emphasize an extreme or surprising case.

Why is there a tan in ¡Qué colina tan empinada!? Doesn't qué already mean "what a"?

In Spanish, when you make an exclamation using qué + noun + adjective, you need to insert tan (or sometimes más) before the adjective. Without it, the sentence feels incomplete or unnatural. So, "What a steep hill!" translates to ¡Qué colina tan empinada! (which literally translates closer to "What a hill so steep!").

Why does the sentence use me duele instead of conjugating the verb for "I"?

The verb doler (to hurt or ache) works just like gustar. The thing causing the pain is the grammatical subject, and the person feeling it is the indirect object (me). So, el sillín me duele literally means "the saddle causes pain to me".

Does sillín just mean a small chair?