You can perform an asynchronous blob copy to make a physical copy of a vhd. See here for REST API details. This operation is very fast within the same data center (maybe a few seconds?). You don't need to make raw REST calls though: There's a method already implemented in the Azure cross-platform command line interface, available here. The command is:
azure vm disk upload
You can also take blob snapshots, and return to a previous snapshot later. A snapshot is read-only (which you can copy from later) and takes up no space initially. However, as storage pages are changed, the snapshot grows.
One question though: why such a large VM image? Are you storing OS + data on same vhd? If so, it may make more sense to mount a separate Azure Drive (also stored in VHD in blob storage) to store data, and make independent copies / snapshots.