One form of training which I find to be almost universally ignored among climbers is sprint training. This is training that literally consists of sprinting. You might not think that this type of training would be helpful for climbing or other sports that don't involve running, but I've found it to be an invaluable component of my training program. Sprint training has the potential to build tremendous general cardiovascular fitness and anaerobic endurance, aerobic endurance, and leg muscle and joint strength. Athletics generally rely on a strong base - legs and ankles/feet, sprint training makes them super strong. But its the general anaerobic and aerobic fitness gained from sprinting that climbers are going to LOVE.
Improving overall anaerobic capacity improves the ability to climb through lactic acid build up in the forearms. This means with good overall fitness, you can climb longer! Not only that, but improving aerobic capacity, which is also trained with sprint training, improves the body's ability to process lactic acid, speeding recovery. This means that the more fit you are, the bigger each and every rest feels.
The goal with anaerobic training is to force the body into overall oxygen debt. Though it is possible to fail on climbs because of lack of overall fitness, its usually the grip that gives out first. Its not easy to force oxygen debt using climbing - enter sprint training.
When I do a sprint workout I generally run for one or two miles to start. Its important to be warm, as sprinting is a near maximal intensity exercise. I'll run fast in bits to get the muscles more loosened up, before starting the sprints. I usually do one or two sets of sprint "intervals". A sprint interval consists of sprinting, interspersed with rests. A good rest to use is a jog of equal length as the sprints. Other rests could be to shadow box, climb a boulder problem, do yoga, etc. Living downtown, I have convenient landmarks with which to control my intervals - city blocks. So I'll sprint one city block, then jog the next, then sprint, jog, etc. If you have access to a track, a good substitute is "straights and curves". I usually do one set of four sprints, and if I'm feeling frisky, I'll rest 10-15 minutes then do another set. The workout might go like this:
* warm up
* jog 1-2 miles
* sprint intervals - 100 yards run/100 yards jog/100 run/100 jog/100 run/100 jog/100 run/100 jog
* rest 10-15 minutes
* sprint intervals - 100 run/100 jog/100 run/100 jog/100 run/100 jog/100 run/100 jog
* cool down
I find that if I do this workout once or twice per month, I have a spring in my step, my joints in my legs feel healthy, and my overall cardiovascular fitness is NEVER the limiting factor of my climbing. If you take climbing seriously, you should be sprinting at least once per month. Given that it doesn't take a lot of time to reap the benefits, there's no excuse! A regular running schedule (with or without sprinting) will always benefit climbing performance...
I will say that one should not jump into sprint training if running is not a regular part of the routine yet. To ramp up to a workout like this, jog 2-3 miles, 3 times per week, for about a month.
This is just one form of "interval" training. One popular, sport specific, interval training routine climbers use is the 4x4.