You are welcome.
To answer your question: I do the 45min as one interval with no breaks, sometimes i take a minute (stop the clock) after the warmup to catch a breather, drink some water, etc.

The key here is to build up to that level (of duration and intensity).
A very general way to periodize things is:
Week 1: Z2 rides, slow steady and easy; get back into it
Week 2: 20min @ Z4 (intensity 0.9 of your FTP)
Week 3: 25min @ Z4 (intensity 0.9)
Week 4: 30min @ Z4 (intensity 0.9)
Week 5: 40min @ Z4 (intensity 0.92)
Week 6: 40min @ Z4 (intensity 0.92)
Week 7: 40min @ Z4 (intensity 0.94)
Week 8: 40min @ Z4 (intensity 0.94)
Week 9: 45min @ Z4 (intensity 0.94)
Week 10: 40min @ Z4 (intensity 0.96)
Week 11: 40min @ Z4 (intensity 0.96)
Week 12: 2x30min @ Z4 (intensity 0.96)

The idea is that you are working near your “redline” (or FTP, lactate threshold). You are still using oxygen (aerobic) and not producing much lactate so that your body eventually supercompensates by pushing the redline higher. It is important to gently build the volume (how long) first and than the intensity (how hard). As a result you will be able to do the shorter durations harder. As you can see after the 45min it is back to 40min, however, the intensity is higher. Some people brak up the 40min into 2x20min with 2-5min rest. The ideal workout is a 60min TT, however, this is mentally very challenging and unnecessary to be performed often in my opinion. Breaking it down in smaller pieces of 30min is doable. There is no single road here where I can do 20min uninterrupted so the indoor trainer is a very helpful tool here. How many weeks and how steady to build up with those Z4 workouts depends on when you want to be at your best fitness. I have done a 10, 20, 30min @ Z4, 4 week build period with great results. (Periodization is a topic I will cover in the future.)

The take home point is that the body supercompensates to what is asked from it. By “asking” the body to push the redline higher you develop a higher capacity (to use oxygen) so to say, I am simplifying here. Therefore you can push harder without going past the redline (and getting tired quickly).

Measuring the intensity is ideally done with a powermeter AND a heartrate monitor. You need to know/realistically guess your FTP (which would be intensity of 1.0) and lactate threshold heart rate (LTHR) (NOT your max heartrate). You should be hovering around your LTHR for the interval and as you get fitter the power output will increase, while the average HR remains roughly the same.

If you do not have a powermeter, you can use a heartrate monitor and perceived effort. It should feel tough and challenging to finish, though not impossible. Remember you should be able to sustain the effort, not hang by the skin of your teeth to finish it. It is simpler to figure out than it sounds. The one caveat is that you will have to find out what is your LT. That involves going past it. How would you know? Once you are past the redline you fatigue very very fast. So with some observations you will find out at what point you were past the redline. You need a ballpark number, so don’t get hung up on the small details. Of course perceived effort is subjective so not ideal, however, it will get you close enough. Just be honest with yourself and do not ask superhuman feats. I repeat myself, however, sustaining and steady rather than bursting.

I hope that clarifies it a bit, if you have more questions you can post in the comments section below.

Good luck!

A question: I’d be interested in more details of your particular workouts – your Monday ride of 45 minutes @ Z4 – was that one long interval, or did you break it up in to intervals, and if so, what?

Spring is coming, even in Ontario, Canada!

Hi and thank you for your comment! I am glad you found motivation in my posts.
Stay tuned for more articles especially once racing starts up (when the weather hopefully starts cooperating). I have just added a “Follow by e-mail link at the top right corner of the blog for convenience as well.

Good luck with your cycling endeavors this season!