"What is the ugliest part of your body?" Frank Zappa once asked. "I think it's your MIND," he answered, after considering your nose and your toes as possibilities.
That is certainly true when it comes to exercise. All of the other reasons/excuses/rationalizations for not doing it are ancillary to the work of the true enemy, your MIND.
So much of exercise is psychological, and this is especially true of running. I sometimes say that running is the ultimate proof of "if you think you can and if you think you can't, you're right." It's mind over matter. It's thinking, "I can't go on," and "I don't want to go on," and "I just want to walk," and "I want to be in bed," and running anyway. Overcoming your mind is the hardest part of the whole process, and it is a process: getting out of bed, getting dressed, getting gear together, getting out the door.
But when I'm actually doing it, instead of thinking about doing it or all the reasons that I can't do it, my mind is my greatest ally. Focusing on pace has helped with that. So has imagining climbing a flight of stairs when going up hill, or spotting a landmark ahead and thinking, "I just have to make it to the fire hydrant. OK, now I just have to make it to that mailbox …" The little victories add up.
Some weeks are good for exercise, and some weeks are not good, but the good weeks always feel better than the bad weeks. On the good weeks, everything is clearer, and I don't have the nagging feeling that I should be doing something. On the bad weeks my mind becomes ugly again and everything else is muddled, too.
But every day, the one thought that keeps me going is this: Even the most out-of-shape person out running is out running! I think they have the most beautiful minds of all.