If you're a first-time marathoner, Dr. Steinberg said there are countless resources online where you can find the best training plan for you — Hal Higdon and Nike Run Club, to name a few — but the most important thing is to have structure. It will all depend on your ability level starting out, and you should aim to increase your mileage about 10 percent per week, he said, adding, "Set realistic goals and know your abilities." He also recommends a training log to track your progress.