They are okay with me, but I would add a couple of hyphens: A 30-minute walk, 30-minute walks. Sentence 4 has an infinitive (to walk) as its subject. I know some people dislike this even though it is perfectly grammatical.
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lucas21cIt's considered bad form to begin a sentence with a number written in digits. It's better to spell out the words.30 minuteThirty-minute walks in the morning are good for your health.