it's good to have more than 1 backup copy, and to make a backup regularly, especially after doing a lot of work on a document.

A disaster-proof backup is on a removable medium. But for near-disasters as this one, a folder on the hard disk suffices to hold as many older versions as you like. If disk space is add a premium, use a zip-program.
For critical work, consider taking a future totally free 1 Gb Google mailbox, and mail each milestone version to yourself. Google takes care of storage and backup.

Working with subdocuments in Word, so that the doc is contained in multiple smaller docs, could also make sense. Losing 300 pages in one time is a lot.

But I'm afraid it's too late now.