Don't over-do it, either. Work out easy - easier than you think you should, but longer than you want to. You will be working in you "aerobic zone" and learning to burn fat, but for this to be effective, you'll need to work out for at least 30 minutes (more would be better). You should finish a workout ready to do another if you had to. This will build your base, and not wipe you out or injure you. Biking to work has done wonders for a lot of folks and it fits the workout in to your regular day. Is that an option for you?