As an injured runner, running always being my primary sport, I have a good deal of experience using cycling to supplement my training. As far as general fitness goes, running to cycling is roughly a 1:3 ratio in terms of distance. In other words, someone who cycles 15-30 miles per day will have similar fitness attributes to one who runs 5-10 miles per day. Although these two people will not have identical musculature, they will perform very similarly to each other in other activities, i.e. swimming. Obviously, the runner will beat the cyclist running, and the cyclist will do likewise cycling.

For someone who is training for a running race, supplementing running with cycling in training can result in equivalent sport-specific performance gains if the ratio is tweaked to emphasize running, say, 6:1 cycling to running. A 60 mile per week runner will decimate a 360 mile per week cyclist in a footrace, but a runner who does 40 miles per week running and 120 miles per week cycling will be quite competitive, perhaps even having an advantage.

Besides attempting to substitute one form of exercise for the other for performance reasons, cross training in this manner is useful for decreasing overuse injury risks. Running muscles, although not primarily engaged in cycling do help reduce load when cycling-specific muscles are fatigued. The same goes for running.

I hope that wasn't too long for people to read.