It's been a long time since I habitually woke up early every single morning. Recently, loving to sleep in, I've mostly woken up late. Today, I woke up really early and feel great again. I can do lots of things that I couldn't have done if I had woken up late. I am so delighted. Use "can" instead of "could" because you want to make a firm statement: you CAN do those things. "Could" works better to express doubts or imagined situations. Your use of "couldn't" is correct because it describes an imagined situation: "waking up late" is something you didn't do, so it is imagined.