Running is excellent exercise. You can get in a good workout in a short time. However, I think the potential for injury is greater in running than in cycling. It is important to take it very slow in the beginning. There are many couch to 5K programs on the internet, but I would go even slower than they suggest. In the first few months muscles develop faster than tendons and ligaments, so you have strong muscles using weaker anchor points and it can cause an injury that will take a very long time to heal correctly. Adequate rest and more frequent runs are more important in the beginning than increasing distance.
That said, running can be used when you need time off the bike but still want to train. It is relaxing and peaceful.