What is the duration of each occurrence? I'm guessing 2 hours. The duration is the span in working time from the start of the earliest occurrence to the end of the last. During that two hour period - I'm guessing you are assigned at 100%? If you assign yourself to any other task that overlaps the two hours, you are overallocated.

An alternative is to create a non-recurring task and set the duration to 20 days. Set the task as Fixed Duration - assign yourself and enter 40 hours of work. Project will assign you at 25% and schedule you for 2 hours per day. The 2 hours is spread throughout the entire day duration. You still have 75% available for other work on the same day.