I see a lot of people have put their 2 cents or more in, so I am just going to offer my 1 penny of advice. I am doing CrossFit right now within my Marathon training program, and I have seen benefits to my speed and to my overall strength. Now, I am not a super competitive runner, so my experience my be different from those who are running marathons to win (overall or age group). My training calls for a normal intermediate running schedule (Hal Higdon) with an added CrossFit workout after my short, 3 mile runs on Mondays and Wednesdays. I have dropped my pace per miles by about 30 seconds and have seen vast improvements in my overall runner's physique and upper body strength. Some runners may not like the sound of that, but I have found that upper body strength helps me keep an upright form when running and has reduced the strain on my back. If you want to check out how I'm doing this, please check out http://mrmirek.blogspot.com
I try to keep this updated as much as possible to keep others informed. Enjoy!