Like Matthew says, the verb tenses must agree (i.e. "am" followed by "will" and "was" followed "would"). But while it has become common usage and acceptable to say "if I was there" to express a hypothetical situation, some people will argue that the correct form is "if I were there". The "if I was" construction would be more appropriate in the sentence "If I was there, then why don't I remember what happened?"

If the person you were saying this to was at the gym and I was not at the gym I would say "If I was there, I would do the exercise with you." or "If I was there, I would do the exercises with you". If I am telling a person that when they go to the gym in the future that I will exercise with them I would say. "If I am there, I will exercise with you.