Hi Sharon. I don’t know if there is a ‘best’ cross-training for runners since everyone is different. But, I do agree with Jocelyn that cycling and swimming are both good. Additionally, I would add weight-training (particularly upper body) and stretching (yoga or otherwise) because in my experience in working with ‘distance’ runners is that these are areas that they do NOT typically focus on.
I hope that this helps.
I do think LaRue is right that there will be variability across individuals training in the same discipline, and that each person will have somewhat different needs.
Buy I also agree with Jocelyn that yoga can be good for some runners. in fact I have members of runners groups in some of my classes. The stretching, particularly around the calfs and hips and legs is part of what is useful, but there are other things the discipline has to offer. This is a blog post I did recently on the subject, if you would be interested:
I agree with the other answers and thought I’d add a few that I believe are also good:
Also anything that gets runners out of the repetitive nature of running straight forward, like tennis, basketball, etc. Doing more activities like this will help avoid muscle imbalances and injury.
The others have given you some great ideas which I also use. It’s true that many runners don’t like to workout their upper body, so what I have my clients who are runners do, is add upper body exercises while they are doing their runs. When the client goes for a run, I have them do a number of repetitions of push-ups, dips, pull-ups, bear crawls, sit-ups, burpees, squats, etc. every few minutes or after a certain distance. For example, if someone is running 10k, every 10-15 minutes or every 1 mile they must do an x number of the above exercises before continuing with their runs. Many of them mark certain playground parks in their routes and do those exercises there (playgrounds are great for many upper body exercises).
In some cases I have them do their runs while carrying some extra weight (10-20 lbs.) to add some resistance into their workouts.