There are many theories regarding cross-training. Some say that cross training is ineffective and only serves to help the individual get their overall fitness level. Others say that it doesn't necessarily help with the running, but due to the nature of training, it will help prevent injuries and strengthen muscles and ligaments to withstand the impact on runs... especially the long runs. Still others say that not only does it serve as a an injury preventative, but will also enhance performance.
I personally believe that cross training to a certain degree will help prevent injuries and to an extent, help enhance performance for endurance athletes. I understand the point of view of those that see cross-training as something that works on muscle groups that do not impact running, thus adding weight, causing the running efficiency to go down. Although this does make sense, I look at cross-training as something that will work those muscles out, but those muscles worked out to a degree will help keep the body in balance. When you sharpen something so much, not only will it become something that can cut easily, but all the same, it is easily broken if hit right.
To me, cross-training allows me to train my muscles that are used to running without the wear and tear constant running might do to it. I personally like to ride the stationary bike, cycling hours on it to work on the same muscles used to run with... but in a different way. Cycling uses more quads than running, stabilizing the muscles, allowing me to grow in strength, also giving my joints a time to recover from the wear and tear I would get from running. I usually don't do anything else other than cycling, but occasionally, I would work my arms out and do ten or twenty pull ups and twenty five or fifty push ups as well as using dumbells to strengthen my arms as they swing, so that they would not just have the endurance, but strength.
Goals can wear you down. When you do one thing and only one thing, it can get you burnt out. You need a variety of things that supports you and your goal. Same goes for a lot of other things, homework, work, relationships... and if you stick with the same thing, you might get tired of whatever you are doing. Spice up your homework by going online and go to sporcle.com and take an educational break. When you're at work, take that break from the harder things you have to do and do some of the simple things to take your mind off and be productive. In your relationship, don't always go out for a dinner and a movie. Go out. Explore. Do something different, hiking, museum... something different that the two of you might did interesting. Add that spice into your life.
Basically, having a variety of things you have to do will help you achieve your goals. Keep your eyes on the goal, but remember to take a productive break, going for your goal but at the same time, giving you a break so you become even stronger when you get off your break and get back towards running to your goal.
Today, I went to the gym and after doing my ten pull ups and twenty five push ups, I got on the stationary bike with a book and cycled for 1:50:00 cycling for 34.3 miles. After that, I did another ten pull ups, and took a break, went to Walmart, bought Pedialyte, and then went back and did another ten pull ups, 335 (25-24-23-22...3-2-1-10) single calf calf raises (per calf) and then 500 calf raises in a row (both calves). It felt do good to do that. Then to test out my new GEL Hyperspeed 4 racing shoes, I ran a 5k (3.1 miles) on the treadmill, finishing it in exactly 25 minutes, an average of 8:04 per mile. After that, I went over and worked with some dumbells, and then finished my workout with cycling 9.63 miles in 40:00 and a 22 minute sauna. It was one long and fun day. I didn't even feel the pain that was bothering me a little yesterday. Cross-training today was a good choice.