A majority of painting is problem solving. Even if we know ahead of time what we will be painting, there are always obstacles along the way. Should this tree be moved? Is this too green? These are the questions that help make a strong painting, although they aren't always easy to answer.
Exercise has had a profound impact on how I solve my problems in a painting. For one thing, it has helped resolve difficult areas a lot sooner. Instead of locking myself in front of my painting when stuck, doing a 30 minute workout allows me to get my blood flowing while thinking about the painting at the same time. Creativity doesn't always happen in the studio. Oftentimes it happens when we are away doing other activities. In fact, most of my “aha” moments happen when I'm about 5 minutes into a workout.
If you are really stuck on a painting, one thing I found that helps is to take a picture of the painting and keep it visible during a workout (if running, I hold the phone in my hand). As soon as you start to breath heavier, take a look at your picture. Oftentimes the problem areas will stick out like a sore thumb.
After my workout, I try to get at least 10 minutes into a painting to address the problem areas. Even if I just leave a note for the next day, I know I have made a lot more progress than if I stayed in the studio all day.