Simply go outside to run a bit. No accounts needed. 😉 Be logged in on the site; then use the deletion link.
URLS:
http://www.myfitnesspal.com/
http://www.myfitnesspal.com/account/confirm_delete
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.