One of the uses for the past continuous/progressive tense is to describe an interrupted action, but that's not what's going on here.
Another, perhaps more straightforward, use of the past continuous is to describe an ongoing activity that took place at a certain time.
When you mention an exact time in the past, use the past continuous tense if the action started before that time and continued afterward. If the action begins at the time mentioned, use the simple past instead.
Past continuous tense examples:
At midnight, everyone at the party was still having fun.
I was working as a cashier in October.
By the age of nine, I was training to be a ninja.
Past Continuous Tense: How and When to Use It, With Examples
The simplest context where this sentence would make sense would be as an answer to the question, "What were you doing at 7:45?" Other, more likely contexts, would probably demand that the example sentence be altered in some way.
For example, if you wanted to describe being interrupted, "I was getting ready to go out at 7:45, when someone knocked at my door." Or, you could emphasize that the activity was going on longer than expected like this, "I was still getting ready to go out at 7:45."