If you feel like you’re drowning, you’re probably at 95% VO2peak or greater. If you’re not used to it the next day your intercostals will be sore.
If you’re going to attempt these extended hard start VO2max intervals it helps to mentally prepare yourself before you start. It’s gonna feel like you are suffocating. Don’t freak out when that feeling hits you. Relax. Keep pedalling. You can do it.