Easy, your 2x20 efforts with 15 minutes RBI are not at threshold. The typical rest for 2x20 is three to five minutes, and one should not be that gassed at the end. Your fitness today may not be what it was a few months back. To use it as a measuring stick, it only counts if you can reproduce it today. Do a proper FTP test first and see where you stand. I like Kolie Moore's test, https://www.trainingpeaks.com/blog/t...ing-protocols/. Sounds like you're new to structural training, but either way, it might be good to start with 10-minute intervals, with a total session time of up to your time to exhaustion, and progress until you can handle 2x20. For me, it's more of getting into the groove of things, especially after a bump in FTP or after a break. Also, keep in mind that any interval less than 10 minutes is a waste of time.