The worst thing about doing it on easy terrain is motivation, but you can certainly work it into your commute. There is no getting out of a hill. You either go up it or you walk.

As far as how worn out to get yourself, that starts to get into specific speed versus distance type goals and other considerations. As a general rule of thumb I would say do a hard day and then an easy day or maybe two easy days.

Nutrition plays a big part in it too. If you are eating well, meaning carbs and proteins at the right times, and in appropriate amounts, you can manage a more difficult schedule.

Don't be afraid of easy days though. It is very easy to get overuse injuries or other overtraining problems, which basically make your progress backward.

Aside from a cycle training book, I would suggest Nancy Clark's Sports Nutrition Guidebook or something else like it. That was a real eye-opener for me, and helped a lot in my training. Helped a lot with the grocery bill too.