We commit to things everyday, usually out of necessity such as: Waking up on time for work or appointments, Getting dressed, taking care of our basic needs and so forth. When we have to be somewhere at a certain time for a prioritized reason we typically make certain it happens, we even put it on our schedule.
Believe it or not, exercise is a necessity as well, if you put it off and neglect it eventually it will catch up with you. Poor health will indirectly effect every other aspect, commitment and need in your life.
In order to maintain a healthy and fit lifestyle to enable you to accomplish everything else, you likewise need to make time for such. Put it on your calendar to work out, dedicate times and days ahead then commit to it, do not put it off or cancel on this appointment for something you deem more valuable unless it is an emergency. Once you recognize exercise as an appointment you must keep like any other than you will find it becoming a regular part of your daily and weekly routine as anything else. If rescheduling is a must then do so, but reschedule! get that appointment still in.