Break the mirroring, bring the mirror online. At this point there will be no more principal or mirror, just two databases on different servers. Providing the mirror doesn't also need a long recovery period, you can then work on the mirror as necessary. If the mirror also needs a long recovery time due to whatever caused it on the principal, then it too will sit recovering for however long it needs.

To re-establish the mirroring, you'll have to redo it completely, right from restore full backup.