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>I'm training for a marathon
I hope you're following a schedule. Don't overwork yourself, and be sure to have easy days each week. It's not a constant climb to reach 26 miles, it's more of a roller coaster of training speed, distance, and endurance. I haven't done a marathon (only a half), but I'm close with people who have, and there's more to the training than meets the eye.
Even if you're just trying to get/stay in shape, you need to look at every aspect of your life for ways to improve: sleeping enough, cutting excessive snacking, constant hydration, stretching, etc. It takes perseverance, but you can do it!