I agree with danowat's point. The cardio work of either will benefit each, but the physical mechanics of each sport don't naturally benefit each other.
If you want to be a good runner, you are better to solely run. If you want to be a good cyclist, you are better to solely cycle. However, particularly for running, if you reach a point where you can't increase mileage due to injury, the added cardio work will be beneficial.
As a runner, I started cycling in the summer due to an injury. Am used to running 70+ miles a week and found riding 150+ miles a week pretty easy. But when I went back to running, although my pace/HR correlation was roughly the same, my legs struggled with the pounding over a longish run (13 miles+).
Fact is, you only improve your running by running and I assume the same for cycling. Although with running, you can ease some of the impact by getting some good cardio on the bike. The 4-5 hour rides (+ other shorter rides) I've done over the summer have acted as a good aerobic base for my return to running over the winter. Although I do intend to keep a long ride in each week to give me that aerobic boost. In summer I will switch back to cycling to have a rest from the impact running but maintain fitness.
Bit of a long waffle there :oops: