You also know for a fact that you don't need to keep them and are happy with your current revision.
So what you need to do is to go to your phpMyAdmin page and do the following.
Type the following into the SQL command line interface:
DELETE FROM wpposts WHERE posttype ="revision";
NOTE: This only works if you have already published your page. (e.g. not saved draft)
After that you should have empty post revisions and therefore the backups for your website should not take as long to accomplish.