Cycling minutes are not as efficient as running minutes. Running will get your heart rate up faster than cycling.

5,000 meter events aren't that strenuous. Its when you get into half marathons that test your body's ability to do a good pace throughout without running out of steam.

Cycling could help running in that aspect. But it would take a lot of cycling minutes to improve aerobic capacity, more than running minutes. The triathletes spend different times on bikes than running. They know the difference and their weaknesses.