Do i have to work out 45 minutes a day at a time or can i do it at different times?

I have so many things to do during the day. school, homework, friends...i dont have time to walk on the treadmill for 45 minutes at one time. can i do it at different times throughout the day and still be healthy? i also need help on a new diet plan. not like atkins or anything but like a home diet plan thats free!