How can I add a personalized symbol to Calendar entries?
There are several possibilities to add symbols to iCal/Calendar.app:


Use Emojis. Emojis are Unicode characters and will be displayed without special software. Use Cmd + Ctrl + Space to insert emojis or any other special character.
Use the find function. Use a unique name for your event ("Exercise"), go to month view (Cmd + 3) and search for your event (Cmd + F)
Use a separate calendar to hide your other calendar temporarily. You will only see your exercise events.


You cannot add graphics to a events' title so it stands out easily. But I think using emojis as the first character for the title could work in your case.

Happy exercising!