Many runners don't realize that some added muscle mass can actually help make them more efficient runners. The other misunderstanding is that unless you're following a hypertrophy program designed to create those bulky muscles, you're not going to turn into "Arnold" by doing resistance training. Using lighter weights and more repetitions focusing on the quads, abs, hamstrings, glutes, calves, shins, and hip flexors will help build muscle endurance. Even doing upper body resistance training can aid a runner, especially an endurance runner. Think about it. Once your upper body gets fatigued, what's the first thing to go? Your form. Once your form is gone, then your performance is going to wane.
To go a step further, functional resistance training (using stability balls, bosu balls, resistance bands, foam rollers, etc.) is even better for runners. Functional training involves working the muscles (with or without) some resistance in a fashion that simulates natural movement. Adding specificity to functional training is an even a better approach. Specificity is gearing your functional training in a manner that simulates movements related to your sport.
The biggest benefit of functional training is that it leads to better muscular balance and joint stability, which will have a more positive impact on your performance as well as decreasing the chance of injury.

The Runner-Core Program provides:
• a Running Specific Core Training and Recovery Workouts DVD containing:
- 3 different progressive 15-minute functional & core strength circuits
- progressive warm-up exercises
- a 15-minute "Fix the Hips - Flex the Knees" bonus workout
- a 15-minute stretching and cooldown circuit
- a Bonus 8-minute core blaster workout
• a 75-minute Audio CD: Coach Al's Five Keys to Fueling Your Best Run Ever!
• a 4-week running plan (spiral-bound book)
The DVD presentation isn't the most polished or "snazzy-looking" exercise program I've reviewed, but that actually makes it more appealing to me. Coach Al is a real guy, a real coach, and a real runner. Somehow watching him go through the various routines is much more believable almost like I'm at one of Coach Al's workout sessions. I kind of like that. When he tells me to "squeeze that glute....hold..." I chuckle, but I actually do it. The camera zooming in on the Ironman tattoo on his calf humors me too, but then I think...hey this guy really is a runner, a hardcore runner. The unpolished feel ends up adding to the credibility.
The other thing that adds to the credibility of the program is that its purpose—using functional exercise to build a stronger core, more flexibility, better balance, and increased strength—is dead-on. More and more research is showing that performing specific functional strength exercises that mimic the neuromuscular demands of running improves running economy, lactate threshold, and resistance to injury better than any other type of training.
The other thing that adds to the credibility of the program is that its purpose—using functional exercise to build a stronger core, more flexibility, better balance, and increased strength—is dead-on. More and more research is showing that performing specific functional strength exercises that mimic the neuromuscular demands of running improves running economy, lactate threshold, and resistance to injury better than any other type of training.
The program sells for $69.95, but I checked the website and it's now being sold for $49.95. I highly recommend you check out the website and consider using Coach Al's program. If you don't think Coach Al's program is what you want, check with your local gym and see if they offer functional training for runners. Coach Al's Runner-Core program gets 5 out of 5 "Dudes" on RunnerDude's new rating scale.