I'm wondering the exact same thing...
Warm-up is more like "getting in the right mood to be productive" for me.... and I often end up on Internet procrastinating. The hardest part is getting on the real work (even if it's just sketching, which is rarely before 4pm...), after that I can stick to it for hours but until then I get easily distracted (garden needs some care, cat needs some care, other cat... so many cats!)