Human memory is bias towards most recent things in a group set. If your set is a “workout” or a “workday”, doing the fun stuff last will affect positively all the memory items in the same group set. This works even if you know that your memory is doing this.
We don’t live in a “present now”. We live in a mental image constructed from memory of recent past.
Trick is not to do unpleasant stuff first, but to do pleasant stuff last.