Longer, higher-intensity cross-training sessions can boost your endurance by providing an additional aerobic stimulus because there's only so much running you can do without getting injured.
Pool running, cycling or elliptical training carry very little injury risk because there's no impact. Pool running and cycling aren't even "load-bearing" since you're not supporting your body weight. Consequently, you can spend a lot more time cross-training without the fear of injury.
While most runners would rather run all the time, runners need to spend more time developing their fitness in less risky and more beneficial ways. Cross-training may be the key to that next personal best.
So, what are you waiting for? Get on that bike and become a better runner.
Sign up for your next race.