Motivation is hard to come by! your in the right place. I'm a new member as well and it's nice to be reminded that there are other people out there doing the same thing as I am. I've heard PX90 is pretty intense. If you start right away maybe it could lead more to frustration than motivation. I would be the type to get discouraged.The more you run, the easier it gets and it'll help build up endurance. something that helps me alot is progress pictures. Take pictures of yourself throughout your process then go back and start comparing!
hope this helps. Good luck to you!