Speed and power are always proportional. When drag dominates, power is related to the cube of speed. When gravity dominates, power is related directly to speed. Train one you train the other, especially when you are talking about geared bikes. Things are a little different on the track where you have leg speed to consider, but that's for a different discussion.

If you have hills, the most effective way to train power is hill intervals. If you are on the flats, time trialing works. In the extreme case, adding friction drag works as well. In that case, if the friction drag dominates (say you are holding your brakes to add resistance), power is related to the square of speed.