Thoughts?
This happened yesterday, already commuted to work and back, started The Hunted but I couldn’t get through it and felt rough before I even started so I started it at 80% and was happily riding along, hitting those levels when it struck me that it was probably junk miles, I’m not hitting the levels demanded of the session so;
Is it better to do a short session hitting the levels, a lower intensity session that moves my legs anyway or scrap it?
Thanks for bringing this up! I’m interested to know if there is actually such a thing as junk miles.
I guess when you’re really fatigued there’s no point in training, but I’d also like to know if you still have some gas in the tank is it worth it to log in some miles, even at zones 2 or 3.
Oooh I like this question … I often struggle with this myself when having to modify my training plan due to other commitments.
It probably comes down to “what’s the purpose of the workout?” … If Angels was on your program to develop your 20 minute power, then Angels at 80% might not put too much strain on that energy system on it’s own, but aerobic development is still aerobic development and you very likely got plenty, especially with your commuting to/from work (although you didn’t specifically mention the duration and intensity of these rides).
As you can see, I don’t really believe in “Junk Miles” … they’re just “miles that aren’t making the most use of your time”. On a day that you’re supposed to train, better to get some training in, than none.
My 2 cents. 
I think you need to consider the goal of the planned workout, and take stock of how you feel. If you can’t hit the numbers, but aren’t that far off and still have the motivation/desire to ride (mental and/or physical), I think it’s ok to ride at a reduced intensity in zone 2 for the aerobic and mental health benefits if that’s what you feel like you want/need. If riding in zone 2 feels harder than usual but you feel like you want to do something, you can reduce intensity WAY down to zone 1 (100-ish watts even) and just do a recovery spin. Flushing the legs out at a very low intensity can be beneficial, and not considered junk miles. If you’re really miserable, both physically and mentally, then you’re probably better off scrapping the ride and doing a yoga or stretching session instead.
It can take time to develop the awareness of how your body feels and what you think you need. But like everything, the more you do it, the easier it gets!
Thank you so much for the reply, pretty much what I thought, that a session thats supposed to be above ftp then at threshold needs to be at those levels or its just not really doing its job. And also like you say get into zone 2 for some fat burn or 1 for some flush out.
It was a strange feeling yesterday, I’ve been riding The Sufferfest for 6 years I think, so I know my mental health and attitude is just as important to getting it done as the actual riding. But I got back from my commute 18km zone 1/2 and just felt low, no reason.
Just to add in I’m recently started on keto to get fat adapted, so I think my body has carb wobbles sometimes. Aiming to do the NCT next year, 7200km self supported across Europe, think I might need that fat adaption!
I always considered junk miles to be neither go hard or easy, stuck in that middle ground of zone 3, where you’re adding fatigue but not really getting much training benefit. If I’m not feeling a session I’d look to put some time in at zone 2.
I’m with you guys… I don’t believe in junk miles as its doing what we love. Just the mental health benefit of completing a workout even at lower intensity is great rather than skipping it if you can.
Dial it down and enjoy.
As Merckx said… “Ride as much or as little, or as long or as short as you feel. But ride.” 
Starting a keto diet will most definitely affect your energy levels as you adapt to it, which can take several weeks. While you’re in this phase, I’d recommend only doing low intensity training sessions. But yes, you should benefit from being fat adapted on an adventure like the NCT. As long as you feel good once you’re adapted. In theory it’s great, however, not everyone thrives on it so don’t force it if you don’t feel it’s beneficial.