The first thing most people say when they decide to get in better shape for the bike is "I gotta hit the weights". Which is beneficial of course, but there is a huge missing link--CARDIO training. Riding a dirt bike, on track or trail, requires a total package of strength, cardio fitness and flexibility.
As you develop cardio fitness your body is also building all sorts of new little tubes, as well as cleaning out all the old ones so they can deliver more energy faster to your working muscles to keep up with the demand longer. Think about when you're terribly thirsty and you're trying to suck water through a coffee stirrer straw--this is your body as a couch potato. Now imagine your water in a big frosty glass and you're guzzling it(and it's running down your shirt)--this is your body as a freakin fire breathing monster. Not only can your muscles get more fuel, they can also get rid of waste/exhaust faster too, you can work harder longer before the lactic acid build up burn comes on.
Another useful analogy to apply to understanding cardio fitness is diesel/NOS. In a nutshell, when you are working steady at a slower endurance level, you are burning fat or diesel--it has lots of grunt and great economy. When you are sprinting you are burning carbohydrates, or nitrous oxide enhanced gasoline--it has a huge burst of power, but burns fast and is quick to fatigue. You know when you are "sprinting", you feel the burn and muscle turns to jello. Cardio fitness enables you to burn diesel where you previously would have flipped the NOS--you can hammer longer, harder and stronger, and have that NOS reserve for when your REALLY need it(not to mention it will just be that more spectacular!).
You can't tap this monster if you just hang out on a cardio machine or run an easy hour, or even depend on circuit training to git r done. You need to make a plan, and understand how to use heart rate zone training. Last year I loosely used the cardio plan information from the RacerXVT site and was really impressed how I improved and what I could accomplish on the trail--as in I could just DO whoops. My cardio and strength came together to just git r done, really it was as easy as twist the throttle, it clicked. Way cool.
Your homework reading
Part 3 will be Lactic Threshold Heart Rate Zones Versus plain ol Maximum Heart Rate Zones and why LTHR Does It Better.