Unfortunately, in the Mail app on the iPad, a mass delete function is only in Trash. So, yes, you'll have to either swipe/delete them one-by-one or use the Edit key to select the visible ones to delete them. Sorry.
If your e-mail is an IMAP or Exchange account, you might be able to go to the web version of you e-mail account and delete them there more quickly. No guarantees though...
Marilyn