Winnershsaint
Guru
Like Rob3rt said. Once you are used to that, make some sessions longer sweet spot (40-90 minutes) continuously, and maybe one/two a week at hard threshold. The principle to bear in mind is continuous increase in training load, so as to promote overload and resulting adaptations.
Your body is very inventive at finding ways of taking it easy, and it's your mind's job to keep finding additional ways of providing training stress.
Thanks for clarifying that. I have started to use this on recommendation of a friend.
http://www.flammerouge.je/.
Currently working on tempo intervals which are currently at 12½ minutes each at the lower end of the sweetspot. Each interval has three minutes recovery time. Session tops out at 3 x 15 minutes which I will do at the weekend. If I am reading things correctly, and I am by no means an expert or anything more than an a very average middle aged cyclist, it suggests that the greatest benefits in increasing FTP are to be found by working at the middle of threshold level suggesting that workouts out that fall either side of this produce diminishing returns on FTP improvement to the point where at recovery pace or at sprinting pace the effects are negligible. It also points out that physiologically working at this level there comes a point where the strain on the body increases while sustainable volume decreases. Effectively saying that working in the middle of the threshold level gives the best returns but at a price. The price being over-training and going backwards rather than forwards.