I've always found that having a target or a race or what have you is the best way for me to get motivated for day-to-day workouts when I'm feeling tired and/or lazy.

I was talking to a friend of mine, who (it turns out) trained for her first marathon with the running instructor I had when he was training for his first marathon, and I told her that I felt like I'd reached a certain level in terms of running comfort and ability just now, and that I feel like I could take it further with just a little more work. I'd really like to see how far that is, rather than risk losing some of what I've gained.