In the hot summer light his floridness seemed heavy and bloated, and but for his erect square-shouldered walk he would have looked like an over-fed and over-dressed old man.
The negative form of the imperative has don't in front of it, like this: Make sure you have a balanced diet - don't overeat and don't eat the wrong foods.
He nibbled and nibbled, and panted, but still, he kept gulping it down till he made himself ill; Yet he swallowed it all, and 'tis easy to guess, he was soon so unwell that he groaned with distress.