I echo all the sentiments above. My commute went from 26 miles return to 40 miles with hills in January. As the week went on, the rides got progressively harder. It wasn't a fitness thing - I was no stranger to long rides. It was just the fact that I was spending so much more time on the bike with shorter recovery periods. Eventually your body gets used to it. I do it five days a week most weeks in all weathers and I have no problem anymore.
But as has already been said, eat right, sleep well, and above all drink plenty of water and reduce caffiene intake. Fine to have a cup of coffee or two in the morning, but drinking it all day will dry you out.
I always have breakfast (porridge or muesli usually), lunch about 1pm, then some kind of snack with some carbs and protein (sausage roll, ham sandwich etc etc) again about 4-4:30pm - If I skip the snack then the ride home is always MUCH harder. I leave work about 5:30. I keep a bottle of water on my desk and drink about 1.5-2 litres over the course of the day.