How would you say 绵延不绝的小山 and 一望无际的田野 in English
You are pretty close. One thing to note is that 小山 is more accurately translated as "hill" and not "mountain."

Both of these contain what could be considered a 成语 (four-character idiom), and both idioms are very similar.

绵延不绝的小山 essentially translates as "endless hills" and 一望无际的田野 basically means "fields as far as the eye can see."

For more literal translations of the individual idioms, 绵延不绝 means "continuing and stretching without cease," whereas 一望无际 means "one look without end."