Is conserving energy something to do on a day-to-day basis, or over the week? I'm about a month into using my bicycle to commute to work, and one thing I've definitely gotten better at is pacing myself to conserve energy, both for the rest of the commute to work, and for the commute back. 

What I'm still unsure of, however, is whether or not I should pace myself on one day to conserve energy for the next. Assuming I ate the same thing, and got the same amount of sleep each day, would I have relatively the same amount of energy each of those days despite a decent back and forth commute?

Today was the first day of my commuting week, and I had a lot more energy (granted, I have been drinking chocolate milk per the advice of bicycles.SE, which may be having a physical or psychological affect AND that my body should be adjusting to the increased activity), so I ended up biking a bit faster than I normally do. Am I going to pay for this tomorrow? 

edit: the commute is 15 miles.
All other things being equal and provided the commute is no more than an hour you should be able to recover in full for the next day. 

I reckon that with decent food and rest you will always recover in time.

That said if the week is long and tiring and you don't get enough rest you may be slower on the Friday than on the Monday.