I'm going to stop doing errands for other people.
Why do you want to stop now?
I'm tired of running around.
While it is nice to do things for other people, it is okay to take time for yourself too.
It's fun to help other people.
You're right, it generally is fun, or at least satisfying, to help others, even with boring tasks.  I can understand why you are feeling tired of being on your feet all day, so it's fine to take a break, and you can get back into it later, if you want.  However, if people are expecting your help, you should let them know in advance that you will be out of contact.