Running is extremely benifical to your stamina, but it does nothing to the muscle groups needed for cycling. I have heard that for cross-training, you should be doing only about 5% of your training as cross-training. The main purpose is not to get good at the sport you are cross training with, it is to rest the muscle groups used in your primary activity while still training and not sitting around and getting fat.
Swimming is another sport good at keeping up endurance, I just find it mind-numbingly boring.
As a rule of thumb, if you are short of breath when doing an activity, and you can keep it up for 20 minutes or more, you are improving your endurance.