this is zero conditionals. both of the clauses are in simple [present tense. and gram.. they're correct, but just to write the correct forms of words.

Your sentences are grammatically correct. They are perfect. They do not need to be improved. However, you can restate them in a way that gives them a more hypothetical flavor: 3) If I wanted to be in good shape, I could eat healthier. 4) If I wanted to be in good shape, I could go to the gym. #3 and #4 are examples of "subjunctive mood". The subjunctive mood looks like simple past tense (except for the verb "to be" which has the special form "were"), but it is not past tense. It is useful for discussing hypotheticals like this. It is usually followed by a phrase starting with a past tense modal like "could", "would", or "should".