Is cross-training worth it? First, I'll define what I mean by cross-training. Cross-training is doing any activity other than your main activity in an effort to improve your main activity. So a runner might go to the weight room and do squats, leg press, step ups, etc. They could also play basketball. It's just doing a secondary activity with the goal of improving your running. I'm being careful to define it this way because my answer to the question is that it is definitely not worth it. No way, no how! Unless you are an elite athlete competing at the Olympics or something, cross-training is pretty much useless. (Yes, useless! Even counter-productive!)
That got your attention, didn't it? All these magazines and popular articles are talking about the great benefits of cross-training, and here I am saying it's useless. And it isn't just useless for running, but for any sport or activity you do. If my main activity is Brazilian Jiu Jitsu, then running will not be a good use of my time. Before I get into why it's bad to cross-train, let me address why professional athletes do it.
Let's look at professional runners since this is a running blog. Professional runners are already logging well over 100 miles of running per week. Usually this number is only the so called "quality" miles too. Add on top of that figure tons of recovery miles -- possible 200 "real" miles per week. Are you doing that? Could you do that? If the answer is no, then why are you wasting time cross-training? These guys are cross-training because they already have 2-3 running sessions per day. They are physically at their running peak. They are looking for something to give them that extra 1% on the end that can help them beat their competition. Lifting weights might do it for them. Maybe swimming. Who knows...but I can say, until you can run, avoiding running by cross-training isn't going to get you to run.
As a new to advanced runner, you aren't looking for that final extra ounce (yes, advanced -- even someone who completed their first marathon still has a long ways to go before they should consider cross-training). You're still trying get this whole running thing worked out. Every time you go out to run, you are improving your neuromuscular pathways. This is the communication between your brain and your muscles. If you have time to run or go to the gym, then run. It's a must! Unless you can go out and run 20 miles without it being a problem, then you are still developing your running. Nothing other than running could possibly help you with that. Once you are running efficiently, your specific running muscles are developed sufficiently, and your neuromuscular pathways are well paved, then maybe hitting the gym can help a little (but still not as much as running).
Now, let's recall my definition. I'm not saying to give up everything else except running. If you do other activities because you enjoy them, by all means keep at it! I'm just saying doing another activity to improve your running specifically is a fool's errand. You may be thinking lifting weights to develop leg strength (aka running muscles) could help, right? Wrong. Can a body builder with significantly more leg muscle mass than even the best runners on the planet go out and run a marathon? No. If they could, is there any hope they could win? No. Training must be specific. This has to do in part with the neuromusclar pathways I mentioned, but it also has to do with developing the muscles in the right proportions. What are those proportions? Well, if you train your running muscles by running instead of lifting, you don't have to know, it's automatically going to be correct.
The final thing I have to say is that running is more than just muscle strength. Cross-training completely ignores this fact. Running is very aerobic and doing the act of running will focus on building your aerobic base (specifically running "slowly" -- I'm sure I'll have a different post to flesh this idea out more). In addition, to run well, you must use good form. Some form develops naturally just through the repetitive act of running. A lot of it is through focused, purposeful practice of good form. This can only be done by actually running.