Marley was thrilled.
The description of Marley before this shows him leaping, spinning, running and pouncing. All of these are very energetic actions that show he is not just slightly happy. The speaker is using an understatement "Marley was a little excited" to emphasis how actually thrilled Marley was to play ball.
They imply that Myra is uncomfortably hot.
In the passage, the setting is described as noon on a sunny day in August. From this description we can infer that it is hot out. When Myra says, "This sun is cooking me," we can infer that she is in the hot sun and can feel the heat as though she is being cooked. She does not indicate that she is in pain or that she is unhappy. It simply means that she is hot.
