It was to my understanding that if you bring your heart rate up too much (enter the anaerobic stage of exercise) your energy source moves to carbs instead of fats as in aerobic. Assuming there is a diet to go alongside this exercise, it would seem as though this would be counterproductive. Granted, this is provided my limited knowledge of physical training.. So I am only asking to further/make changes to said knowledge.

As for fixed.. I started on a singlespeed, at one point in time my freewheel went ka-boom, and I had to ride fixed until I could get a new one. I ended up hating the freewheel because of the pawls and stayed fixed.