So much for my "Run long on the weekends and you can skip the days in-between" theory. As this article points out, you quickly lose conditioning without regular runs. Some more eye openers from the article:

• . . . training is exquisitely specific: you can acquire and maintain cardiovascular fitness with many activities, but if you want to keep your ability to row, or run, or swim, you have to do that exact activity.
• . . . people who work out sporadically, running on weekends, for instance, will never reach their potential. [this we all knew but I hate to see it validated]
• 'When training time is limited, Dr. Coyle said, “you have to decide where you will get the biggest performance bang for the hour you spend.” The key, he found in his research, is to substitute intensity of effort for time. “A runner who’s been running doesn’t need much time to maintain his performance,” Dr. Coyle said. “But the training needs to be almost like racing.”'

• But the good news is that it takes much less time to regain fitness for a specific sport than it did to become fit in the first place. Even exercise physiologists are surprised at how quickly the body can readapt when training resumes. Almost immediately, blood volume goes up, heartbeats become more powerful, and muscle mitochondria come back.