When I was injured last year it became painfully clear that while I had well developed my endurance and aerobic fitness as well as the muscles responsible for running I was severely undeveloped elsewhere. Once my hip muscles stopped firing I strained my groin while stupidly still running and in a cascade of body failure my legs simply stopped working. We all have a weak point in our body and for me it's my left hip, which is aggravated by a weak bum and tight hip flexors.
Now I'm not going to say that the gym solved all my problems, it didn't. Actually I give more credit to swimming and a great doctor, but what the gym does is give me a more well rounded level of fitness. While I have absolutely no intention or desire to fall into the seemingly ideal male body of huge muscles, I have come to stand behind the idea that endurance athletes need to work on the muscles that don't fire or work hard while participating in their sport. If you're a runner like me then I'd suggest it would pay to see a trainer and get a specific program for you, probably that focuses on core, glutes, adductor and abductor muscles and even some upper body strength.
Like most people nowadays I spend a fair bit of my day in front of a computer or sitting. This lengthens out some muscles and tightens others, usually for runners in particular taking away the bum muscles that could be lending a helping hand on your workouts. Exercises that lend themselves to improving posture are great as well as we all tend to roll our shoulders forward when we get tired on a run making it even harder on ourselves than necessary.
For me, my gym workouts are short (usually 30-40 minutes), in my home so I don't have to drive anywhere and don't involve any heavy weights. They are largely using my own body weight and minimal equipment. Having a proper gym space though I did buy some things such as a kettle ball, some mats, chin up bar, plyo boxes, and some balance discs. I'm not a certified trainer (though at one time I started the course without finishing it) and I'm loath to give specific fitness advice but I will say that you really don't need much. A small space in your home should be adequate with a mat, and a few inexpensive items. One thing that is helpful is the TV and music that I put in the gym. It makes it more bearable and helps me fight my natural resistance to working out.
As an attempt to keep myself healthy as a runner I swim, cycle, do gym workouts, attend yoga and I've recently signed up for boxing classes. All of those come and go as I have time (or don't) and as my interest peaks and wanes. Of all of them I'd say that occasional gym workouts and swimming are the most helpful for me. Yoga has been wonderful too, and I'll be writing a post about my yoga experience soon, but I feel it's actually most helpful after a training period (like for me at the end of 5 months of marathon training) just to reset the body from months of endless pounding.
I have a few books on workouts for runners, and some of them are good, but other than the Cool Impossible which I've adopted into my routine I'd say it's probably best to spend a little money and go and visit a trainer or have them come to you. Make sure you find someone familiar with endurance athletes and their needs. There is no reason to join a gym and inherit endless membership fees. Keep it simple. Hopefully this helps you avoid injury, and you never know, maybe one day you can have your own embarrassing gym photos.