Pretty normal. It’s definitely more intense than just getting out riding (unless your usual ride is a crit). A good way of thinking about it is that you are swapping duration for intensity. It’s possible that you aren’t used to the intensity level, at least not for the durations of the intervals.
It does work - see the FTP improvements thread - I’ve had a 13% rise in FTP over the last year on the Low Volume plans for example.