No matter how much you try to plan your next painting, there are always areas that are more difficult than others. Not just in technique, but also in the places where we get stuck and aren't sure what to do next. This is actually one of the reasons that I love painting so much. If it was easy and we knew exactly what to paint, it would have become boring years ago. But even with these anticipated struggles, one thing that can be very problematic with painting is overworking it to the point where it is no longer interesting.
When I make sure to work out on a regular basis, a lot of times I get ideas in the middle of the workout. They can either be ideas on a new painting, or ideas on how to resolve an issue on a painting. Generally the more I get out of our comfort zones, the better the ideas become.
One of my favorite things to do is to bring a picture of my painting with me when I am working out. Whether I am running with the image on my phone or bringing a print with me to the weightroom, there is something very interesting that happens. The point when the exercise gets very difficult is often the times our minds start to look for ways to push through. Just glancing at the painting at this point is so incredible because the parts that aren't working will stand out like a sore thumb. Now you can feel good for not only pushing yourself in your workout, but also resolving a difficult area in your painting.
If you haven't tried working out with an image of your painting, I highly recommend it. Even if it is a completed painting, you may be surprised to find an area that isn't working.