What can I do to help me sustain frequent long-ish rides? After nearly a year of commuting by bicycle to and from work 2-3 days per week, I have recently started cycling to work every day, 5 days a week, rain or shine.  My commute is about 11-15 miles each way (depending which route I take), including some steep hills (followed by stop signs, so I don't even get the benefit of coasting down-hill). I obtained a new road bike of decent quality along with reasonable quality clipless shoes, gloves, shorts, a jersey, and a heart rate monitor.  I am not very physically fit (5'9" 200 lbs), but improving.  Now 2 weeks into daily commuting plus occasional weekend riding, I am feeling very sore and fatigued and can barely sustain 80% of the effort I was achieving at the start.

What should I eat (and when should I eat it) to help me sustain this effort, yet also help me lose weight and improve my physical fitness?  Is it important to ride at a relatively low effort level, keeping my heart rate in the base zone, or should I occasionally push it into aerobic endurance or even anaerobic zones?  What heart rate(s) should I aim for? What cadence should I try to maintain?

Any advice from experienced commuters in a similar position would be greatly appreciated.
Just keep at it. The human body is very adaptable and you'll soon get fitter.

Don't worry too much about food and nutrition at the moment, that only matters when you're cycling at a competitive level, you just need to make sure you've had breakfast before you ride in the morning. Regardless of cycling/weight loss you should try and eat a healthy balanced diet anyway.

Don't worry about heart rate and zones. At the moment you just need to get to the level where you can comfortably manage 5 days riding. This will come naturally. Its hard at the start (as you are experiencing) but it will get easier every week.

In a month or two you will be able to manage the ride no problems and only then should you think about pushing your performance. That's when the heart monitor is useful.