There's a trade-off mark, with regards to the optimal level of energy derived from fat, during aerobic activity (cardiovascular training).

At lower levels of intensity, your body does derive a higher percentage of its energy from fat over glycogen - however - as you're exercising at a lower intensity, your body requires less -overall- energy.

However, at high intensity, your body uses a predominantly glycogen driven energy derivation - where you require a higher level of energy overall. This is where you may see the trade-off for optimal intensity.

We all have various levels of intensity - because we all perceive exertion differently, and because we're all at various levels of fitness ... therefore it would be remiss of me to discuss a specific "setting". However, as a general -level- of -intensity-, aim for 60%. Your maximal effort is 100%, so roughly gauge your '60%' effort, and utilise this as your "speed" of 'walking' which likely will extend to jogging or running again, depending on your level of fitness.

It's important to be honest with yourself. Intensity often has direct correlations with your actual training gains.

As for your incline requirements - you may use this if you wish ... however, as an incline will add difficulty, you'd likely have to decrease your speed to maintain the same level of intensity - (if you were jogging on a flat setting, you may wish to speed walk if you're on an incline).

While some cardiovascular training can possibly assist - it greatly depends on the trainers goals - where a bodybuilder has absolutely no requirement or necessity to training their aerobic system. Q
uite simply, they're after pure hypertrophy which in effect, will have strong impact on their body fat levels. The key for their body fat regulation actually stems from their nutrition / dietary factor, in conjunction with their heightened metabolic rate due to increased muscle.

As for the "in theory there are some basics" - It is not theory, it's proven, practical and legitimate. You may ask different trainers and achieve different responses - though this does not at all indicate more accurate, or applicable responses.

For example --> You can ask a number of people regarding nutrition, and I'm certain there'll be "someone" out there who'll suggest that a Lemonade Diet, or Low CHO diet would be useful for weight loss - when in reality they're both terrible options.

Of course there are "different strategies" to reach the same (or similar) result - that is the joy of exercise science and the ability to apply it in a range of ways - however, there are certainly optimal or ideal solutions that would 'best fit' ... (even though some body types are better suited to alternate methods).

While you may claim to already be 'aware' of much of the 'basics" - (even though it's actually not that basic), you must realise there are others in the group that may benefit from extra information - hence the reason some of this additional information was included.

Lastly, to address something from your original post - the inference that "fat burning" doesn't occur in the first 20-25 minutes is completely untrue. Beyond the simplistic view that energy in -v- energy out dictates body fat levels, you're using energy immediately, and depending on the intensity level(s) will ofcourse "burn fat" immediately. Again, dietary and nutritional factors are a heavy influence on your results.