There’s not a way in-app to see all test cases within a project grouped or sorted by when they were most recently run. I’ve done a little digging and there are some reports that can help identify when specific test cases had results added to them within a specific timeframe, but the most useful one for identifying cases that haven’t been run in a while would likely be the Comparison for Cases (Results). This will show a breakdown by section of results that have been added to each test case in a project for user-specified test runs. It will provide the info in a grid style showing the latest status for each case in the test run, and you can easily see which are untested.
I know that may not be ideal for easily selecting the untested cases for future runs, but it should help get a better idea of what cases need to be run. Beyond that, it would be possible to use the API and scripting to find the untested case IDs, and potentially feed it into a new run.