Mac needed to work out. He was trying to hide his anger from Gretel. Mainly because he wasn't angry at her. He was angry at the situation. He couldn't believe that they were given a chance to be happy and then it could be taken it away. So he had to work out. Working at the barn could only help so much.
When he got to the gym, he noticed Alex as soon as he walked in. He'd seen her around the building so he was familiar with her. And he knew that she was a ghost so what she was doing didn't really surprise him.