Uhh if you've registered an email address to your Runescape account, chances are your username is stated somewhere within the confirmation email they've sent you. If not, idk wtf to tell you, not very smart.
How on earth can you forget your username? Anyways, I suggest looking through old documents on your computer or any old paper you got somewhere that you may have wrote on, you might find your account details somewhere. Apart from this there really isn't anything you can do so goodluck.
Send JaGeX an email/query. I used sent something similar under billing queries. Just make sure you apologies for using this inappropriate way of communication and be polite