Trying to do both fast and any intensive exercise in close proximity seems counterproductive. It takes a fair while to refuel. So it all depends on how much of the tank you empty. Extending the (partial) fast after a ride will just lower the glycogen stores further making it more difficult to refuel quickly. AFAIU you are not doing it long enough for any significant long term fat adaption so it only serves to mess up the refuelling for any higher intensity workout. So personally I’d not do any fasting on a x2 day unless they were both low intensity and then I wouldn’t bother with any significant carb refuelling anyway.
It seems there are lots of ways to approach fat burning
- Lower the glycogen stores for an extended time, low carb consumption ie. fat adapt your body
- Do lower intensity work and burn it as a % of the fat/glycogen mix - with or without fasting
- Do HIIT boost your metabolism and potential fat burning
- IF and ensure ride intensity matches the IF regime to maximise fat burning after glycogen depletion
- General Calorie restriction
Fat adaption really does seem to be a different thing all together - teaching the body to run on a different fuel and takes weeks of continuous low glycogen. And (AFAICS) it is just for real endurance devotees.
I could be wrong - but I feel you only become partially fat adapted on IF, i.e. you become less dependent on carbs and possibly make more of the carb boost when ingested and it may be the case that you burn a greater % of fat than you would without IF.
I’ve tried to be more fat adapted - I lost weight, got used to running on mainly protein and fat (improved endurance fuelling) - but just couldn’t do the intensive stuff on the trainer or outside. Hills of any significant kind were truly horrible and higher intensity workouts just made me want to stop cycling altogether.
Conversely with IF I get the weight control, ability to still do a range of exercise intensities - but no great sense that you can ride any further without regular carb fuelling. 50 miles is the longest morning ride I’ve done on a cup of black coffee and I don’t need to go any further that often - so why go through the torture of a low carb regime with all its limitations. If I was doing regular centuries or a significantly long distance ride it might be a different preparation story.