Uniform running may result in injuries that will take a long time to heal. Cross-training will help you strengthen your weaknesses. For example, long runs are usually very strenuous, because your legs and feet are not used to repetitive and prolonged impact. But, a long run may be replaced by a long bike ride or hike instead.
The main purpose of long sessions is to improve aerobic endurance. Many runners prefer the long runs to continuously improve impact tolerance. But impact tolerance is better improved gradually to avoid overtraining.
For many runners even recovery runs are demanding, because running always places a large amount of stress on leg muscles. Swimming and fitness walking are excellent for recovering. Your muscles will simply be in much better shape if you strain them in a diverse manner.
By cross-training, you maximize your performance during the all-important intervals and even-paced runs because cross-training ensures your legs are well recovered. Swimming and walking strain the body more evenly, and improve aerobic endurance and oxygen uptake capacity.