Making a runner faster is much more complicated than making a cyclist faster. Here's why:
There was a thread yesterday asking why cyclists are typically pretty good at sprinting but runners are not. The answer lied in economy: the mechanics of a bike largely make everyone ride the same way, there are not different forms. Now imagine the variety of running styles. That's an extra component in running that isn't there in cycling.
So in running and cycling alike, you have your engine, which is basically measured with VO2max. But in running you have to convert that engine into motion, which is where economy comes in. Paula Radcliffes engine got smaller over the course of her career, but her economy improved and she became faster.
So running is not as linear as cycling when it comes to doping effects. You could dope someone to the gills and they could be naturally talented, but if they run down the street like a wiggly goober they are wasting all that horsepower.
Doping is only part of the equation.