My suggestion to a new person in training is to take the first week and focus on dieting. An interesting thing my athletes experience is the obvious effects their diet from as recent as the night before to as long as a few months on their ability to engage and keep stamina with all exercises. Some easy to avoid foods that will cause fatigue, drowsiness, cramps and bloating while training are soda products, chips, fried foods (fries, fried chicken, fast foods, etc.), too much white breads and pure white sugar. Of course, if a person has diabetes I or II they should already be on a plan with a personal nutritionist and can scan through this section. So for the first week, without lifting a weight, I would suggest to take these foods out of a new person in training's diet completely. I would strongly suggest that the person drinks large amounts of water (which helps lean out the body, drinks carrying sodium and sugar do the opposite), eat lean meats and many different colored fruits and vegetables (the more colors the wider variety of vitamins!). Bananas are great but are known to cause constipation with consistent over use. When training begins sports drinks are only to be used during or immediately after exercise, otherwise only water. When coming off of eating processed and sugar based foods, a person may get several fits of cravings, this is OKAY, they will PASS. A weird phenomenon is that once sugar is out of a persons diet, all foods start to taste significantly better (due to several reasons, a basic reason would be the coming down from endorphins created by sugar based foods that make our cravings seek more sugar for more pleasure) and it forces a person to try new foods and cooking styles. For stamina and protein, I encourage reading up on Super Foods (Chia seeds, avocado, kale, etc.).
So why lift weight? Well, if you are thinking of running a marathon but have never trained or have not trained in a long time, there are a few basic reasons to consider starting your journey in the weight room. Consider this: muscle weighs more than fat, muscle burns more calories than fat, so the more muscle a person has, the more they naturally burn and the 'easier' it is to keep a lean body type. Something to remember: low weight/high repetitions yield lean muscles, high weight/low repetitions yield bulky muscles. Running is the best workout you can do for your core muscles (abdominals, hip flexor, quadriceps). Also, your abdominals and calf muscle tissue fibers are different from your other muscles such as your chest or biceps, so in order to improve these muscles you must do very high repetitions on a consistent schedule. On lifting only days moderate amounts of caffeine may help keep focus in the weight room but for many reasons should not be used for long distance running days (if your heart is beating too fast, WALK!). And make sure you sweat, sounds gross, but its one of the best ways to clear out your system.
My final two pieces of advice are to stretch and to periodically take pictures of yourself. It sounds weird, but keeping a log of pictures lets you know how you are progressing in a very productive way. I suggest once a week and to keep it to yourself. Stretching-wise I will suggest to stretch every night, training or not, grab a medicine ball (or basketball) and a smaller hard ball (golf ball, baseball, field hockey ball) and engage in deep tissue stretching. Here is an explanation of what that is by Chris Gizzi, a former NFL player and Air Force Reserve: http://www.youtube.com/watch?v=NLxD-G-MfMI&feature=plcp