30 Minutes of cardio is great and never a waste of time. If you run or bike shortly after you eat it's not the best thing for your digestion because your stomach and muscles are competing for blood. The best routine would be to exercise first then eat. If your goal is weight loss then exercising first will slow down your appetite and you won't eat as much. That said, it's better to eat then exercise rather than not exercising at all.
Relatively speaking, exercising before or after you eat burns the same amount of calories and has the same impact on weight loss. Go for it!
