I used to hugely underestimate the time I need for my tasks and frankly, I still do sometimes. Practising timeblocking has helped me stop and think about exactly how much time I would need for a task.
For example, I allocated an hour to script and plan the shots for this YouTube video. However, the truth is I had to spend 30 minutes just to get into the groove of writing. Then, I took another hour to write the script. Finally, I used another 30 minutes to plan out the shots for the video. Talk about expectations versus reality.
Timeblocking on my calendar app allows me to move or extend the time blocks with flexibility, according to the actual time I spent on each task. I am then able to create my to-do list in a practical manner without overloading it.