Not as easy as one would hope.

Depends on a lot of things. I assume that you cloned the entire disk and not partition-by-partition. That being the case, assuming that your root filesystem is not the last partition on the disk (by geometry), it's probably going to be easier to start over, if you can.

Basically, the idea is that you first need to expand the partition the filesystem is on, then you need to expand the filesystem itself. Expanding the partition means that you have to have free space directly after the existing partition. If it's not the last partition on there, then that means you have to move the other partitions, and that means nearly as much data writing as starting over, with the added advantage of doing things in kind of a hacky manner.

Before I go any further, let me know if you don't have the original disk anymore or if there's some other reason you don't want to start over.