I'm really bad with motivation - I'm one of those people that believes you need more than motivation to get you through this.
Just make a schedule, and stick to it - give yourself no choice in the matter. Make exercise and cooking healthy meals part of your daily routine (or cook on the weekends for the week).
I think if you incorporate it into your life, you get used to it, and its how habits form.
And maybe some days you'll be more "motivated" to work out longer than others