If you think of exercise as 30 minutes a day 3 times a week then diet is by far more important. But, at extreme levels of exercise it's hard to eat enough calories to keep from losing weight.

At the true elite level Michael Phelps is fueled by 12,000 calorie a day diet, eating that much food just hard. Think 60 Twinkies a day plus a 3000 calorie diet.

But even 1/2 that and most people would still have problems keeping up for months at a time. The average human body could do 8 hours of exercise a day 7 days a week for years after training. So we don't really need to look at the true extremes of human capability to find a level of exercise that will balance all, but the most ridicules diet.

PS: If you are going to work a desk job, then nights and weekend exercise is not going to cover a horrible diet.