These sessions are designed for people with minimum time but want maximum return with minimum fuss or set up. Ideally working to Power but if not Heart Rate works fine.
For these session all you need to establish is what your FTP / Heart rate is. If you do not know this then you can do this simple (but hard) test. 20-minute time trial after which 5% is subtracted from the average heart rate or power. This will then be your working FTP but will need re testing maybe every 6 weeks.
FTP stands for Functional Threshold Power, which is commonly defined as the highest average power you can sustain for an hour, measured in watts.
*Please note that many of my sessions are based at FTP minus 50 or 60w – this should be close to Ironman Bike power – but this may need adjusting slightly for individuals.