Base training is traditionally done at a moderate intensity. Spring last year the vast majority of my training was basically riding Audax. My functional threshold increased by 30W and my fat burning percentage doubled.
was that training at constant HR or constant speed?
The former is of course a progressive method where as you get fitter you go faster. I've done it for years for both running and cycling, but it takes discipline.
Constant HR (within reason). I was trying to keep my HR below 140. I was riding fixed, and sometimes on hilly events it would be in the 180s. I was not stopping for long at controls, generally not eating their food, and instead eating home made rice cakes (as per the recipe used by Team Sky) while riding. This approach kept my rolling average down, around 22kph, rather than the slightly higher average I'd usually aim for. As I tended to spend less time stopped, my time around events was roughly similar to my previous approach which was to ride harder and require more feeding.
I got hold of an electronic trainer in early summer and did an 8-week build programme from then until just before PBP, so adding in a lot of higher intensity work, while still riding longer events. My FTP only increased by an additional 10W over and above what I'd achieved by riding Audax. However, my fat burning at moderate intensity actually got a bit worse, though it was still better than it had been in February.
This winter as I have the trainer, I'm trying something different and using a mixture of sweet-spot and base-level work. It will be building on last year's work. However, I don't think I'm markedly fitter than I was in June having just done virtually nothing but Audax riding.