What can a distance runner do outside the weight room to increase fitness?

Other than doing weight training circuits in the weight room, distance runners can do core circuits to improve their core stability, posture, and strength.  Medicine ball training is a great way to improve total-body fitness, strength, mobility, and power by strengthening the muscles, tendons, and ligaments throughout the body. Hurdle mobility (walking over and under hurdles) is a great way to improve range of motion, balance, and stability, while strengthening the muscles in the lower back, legs, and hips. General strength exercises, such as push-ups, lunges, and burpies also help improve total-body fitness, strength, and flexibility.

In addition to the miles that you are running weekly, you should try to mix cross training into your schedule.  Consider these activities as a great way to compliment your running.   To leave your comfort zone can be a scary proposition.  You are a runner and you know how to run.  But hopping into the pool to swim laps may be way out of your comfort zone.  Where running is a significantly high impact activity, swimming is the polar opposite.  You'll be able to enhance your total body fitness by lengthening and strengthening your muscles, tendons and ligaments.  All these advantages without the stress of pounding the pavement.

Also, by adding swimming into your schedule, you might be able to expand upon your running by adding sprint triathlons which include a quarter-mile swim, followed by a 12 mile bike ride and a 5k run.  Most importantly, you should enjoy every step, stroke or pedal that you take!

2 things come to mind. 

1.Cross training for improved cardiovascular results, while limiting overuse injuries from too much running.  (Bicycling, swimming, aerobic classes)

2.  TRX Suspension training.  You'd be surprised how strong you can get using the TRX and your own body weight. 

As a former distance runner myself, I can tell you there is plenty to do outside the weight room to increase your fitness. One of my favorite partners to running is swimming. There are so many ways you can get fit swimming. You can swim laps and you can also swim in a recreational pool with your family and going up stairs to a slide and sliding down the water slide and climbing up the stairs right away over and over again will keep you quite fit. Sliding down the slide also can release endorphins and dopamine just like you get when you are on a final sprint to the finish line in a race and that, as you know increases fitness levels as well. 

I also recommend stopping along your running route at a park or side of the road to do lunges and pushups. You can also do squats and plyometrics and pull ups on the monkey bars. V- ups on the corner of a bench can keep your abs strong for your running efficiency as well.