To clear up a common misconception - Lactate is a fuel source, and is NOT the source of muscle burn/fatigue. As you exercise, glucose is broken down into pyruvate and lactate. Pyruvate is used in the mitochondria to produce ATP (Adenosine Tri Phosphate), which is what produces energy in the cells as they break a phosphate bond to produce ADP (adenosoine Diphosphate).
As the cell gets to capacity on how much pyruvate the mitochondria can process, the body correspondingly seeks equilibrium and produces more lactate. It is this buildup of lactate that was mistakenly attributed as the cause of fatigue/burn in muscle. The mitochondria have the ability to take up lactate, convert it to pyruvate and then to ATP.
So as far as muscle burn, physiologists are somewhat back to square one, in that they are seeking what causes it. One of the recent suspects is the buildup of hydrogen (H+) ions which creates an acidic environment in the muscles during high intensity exercise. [Study synopsis]
As far as the fructose causing higher lactate levels, this study suggests that within a few hours up to 1/4 of the fructose can be (not necessarily will be) converted to lactate, but again, lactate is a fuel source and is not the cause of your muscle burn.
Fat metabolism is not an either/or thing. You are always burning some proportion of fat for energy when you exercise. At lower intensities, this may be as much as 50% of energy is coming from fat stores, while the higher the intensity the less you rely on fat. However, overall you generally burn more calories (and thus more fat) at higher intensity exercises. Fructose isn't really going to have much of an effect on this (That I am currently aware of, anyway).
TL:DR; - Fructose won't affect fat or the lactate burn. To be good at cycling up hills you want a good power to weight ratio, and to reduce fat simple calorie restriction on a good diet combined with exercise is all you need.