first off, you more or less have to decide which technique you will use. i started out with one pedal and played it with the weight of the whole leg, thus getting powerful shots. the drawback is that if you try to play like this faster you have to build a lot of muscle because it's a very strenghtening technique.
there are other techniques which require less strength, but are more difficult to learn, such as: to stab the pedal with the middle of the foot, quickly let it spring back and kick it down again with the peak of your foot. combined with 2 feet this can be really fast. there are several other different techniques.
but back to the point. i found out that you do not need to train your feet that much, it's the brain which needs most of the training. so you can do exercises without a drum pedal, for example while on a train or watching tv. just make this exercises part of your everyday life, and allow yourself some rest in between "real" drum sessions. after having played no drums for 3 months, i actually got better when i sat behind the kit again. i believe i just played more "in my brain".
hope that helps a bit