Q: I'm training for a marathon. I would like to strengthen my core but don't know what days to work on that. How often and for how long is it good to cross train?
Just like running, you want to weave in core work gradually and at an effort or volume that doesn't affect your running. A great way to do this is to start doing core work 2-3 times per week on alternate days (M – TR or M-W-F) Great exercises include planks, single leg lunges, push ups, hip extensions (glutes), lateral leg raises. Think of it as a spice in your routine rather than an ingredient. A little goes a long way and the more consistent you are with it, the more it will help your running performance. Here is a simple strength program to get started: http://askcoachjenny.runnersworld.com/2011/12/the-minimalist-guide-to-strength-training-for-runners.html
I also have a full program in my book Running for Mortals. It is also helpful to work with a trainer to develop a program too. Lots of ways to get it in. Good luck!
This question is an excerpt from our Live Q&A with Coach Jenny Hadfield. You can replay the live chat event here. The training plans mentioned in the live chat are available here. If you sign up for a free account on WalkJogRun.net, you will be able to track your training and add workouts in your training diary. The next live chat is being planned for March 2, 2012 at 12:00 PM CST.