I did the same thing, I started riding into work instead of taking the train from the suburbs to Central London. Door-to-door by train and underground was around 1 hr, but bike it was 1hr 10mins, approx 18 miles. Wasted time became exercise time and I felt much better for it. I remember my brother saying he would work with people who would work crazy hours (doctors mainly) and in order to train for marathons and triathlons you utilise every bit of time you can to exercise.