Time Machine on New Mac

I got a new Air back in January (currently running 10.7.5), replacing an old Macbook that crashed. The old Macbook has been backed up via Time Machine to a 1.5 T external hard drive.

I'd been keeping the old Time Machine around, in case there were things that I needed to restore, but now that the new machine has been running for several months, I don't really need the exhaustive files. Still, just to be safe, I would like to keep the final backup of the old machine.

I now would like to use that hard disk to back up the new machine-- but there isn't enough space to do that, because the backup of the old machine is taking almost all of the space, nearly 1.5 T of storage.

What I need to do is delete all the old backups except the most recent. As far as I can tell, the only way to do this in Time Machine is to delete them one at a time, and with each one, answering "yes, I really do want to delete this" and entering my password. There are hundreds of backups, and this would take forever to delete one at a time.

Is there an easy way to do this?

 

Looking to delete all but most recent

Unfortunately, no-- that video shows how to delete one hour's backup, but to delete everything except the most current backup, I would have to repeat that series of actions several hundred times, to delete each individual backup for several years, each time typing in my admin password. That would very likely take several days of my time.
What I'm looking for is a way to delete everything except the most recent backup.