Everything I've read indicates that breaking exercise into chunks is just as effective as doing it all at once. In fact, you get the added benefit of boosting your metabolism several times during the day!
From the latest government recommendations on exercise:
The barrier often given for a failure to be physically active is lack of time. Setting aside 30 to 60 consecutive minutes each day for planned exercise is one way to obtain physical activity, but it is not the only way. Physical activity may include short bouts (e.g., 10-minute bouts) of moderate-intensity activity. The accumulated total is what is importantóboth for health and for burning calories. Physical activity can be accumulated through three to six 10-minute bouts over the course of a day.
Yah what Meg said! I read somewhere about doing short bursts of cardio is cool.
PLUS if you do 15 mins of high intensity cardio, and another 15 minutes later on, you can put more effort into that 15 minutes than you could for 60 minutes, so personally I think you would probably burn more calories. Say you ran at 8mph, but could only keep it up for 15 mins, not an hour, but then later on you run at 8mph for another 15 mins, then that adds up to 30 mins at 8mph. KWIM?
I think you only need to do a long version of steady state cardio if you are marathon training.