No comma is required in the first sentence. In fact, the comma you have seems to put a bad break into the sentence.
In the second sentence, the comma is OK, because it doesn't hurt; it comes as a natural pause in the rhythm of the sentence (which could be two separate sentences, or two clauses joined by "and"):
I'm at the park and trying to get some exercise.
I'm at the park. I'm trying to get some exercise.
As always with commas, their use can't be proven like a math formula. Some just seem better than others. We use many times fewer commas today than in previous generations.