You can do what you do. Its a simple rule, but in this day of "cross-training" a lot of people forget. You will not be a runner if you are always on the bike. You will not be a cyclist if you are only running.
Back about two years ago I was training for a triathlon. I knew that as a runner, my running was going to be my bread-n-butter part of the race, so I focused on the swimming and the biking. My running mileage for the three months going into the race totaled about 50 miles. My swimming and biking had been the major part of my focus and I had incredible numbers.
On the day of the race, I was really pumped thinking I could actually do the event (a sprint) in under an hour. As it turned out my swim came in at 12 minutes, the bike in under 30 minutes, but...the run? Don't ask? The reality was, I couldn't run. Because I hadn't trained to run, my body adapted itself to the swimming and biking. Our bodies adapt to what we require them to do...running in the tri was not there because I didn't put it there to start with.
What this experience taught me: focus on what you want to do. Not parts but the whole. If you want to run the mile race, focus on the mile race. If you want to run a marathon, focus on the marathon. If you want to do a triathlon, you have to work all three areas of the event...don't count on a natural strength in a particular area to pull you through.