Hello.
First, lactic acid buildup isn't a bad thing. If you have a good amount of lactic acid (lactate) in the blood, and you can increase your oxygen intake, then you've got a LOT of potential to make a lot more ATP for your workouts, so keep that in mind when reading the rest of what I have to say.
Going down to the molecular level, we all know that oxygen is necessary to make muscles contract so we can do work, perform our exercise, etc. Why is that? Because ATP, the energy that the body uses to make those muscles contract (produced in small organelles in the muscle called "mitochondria", which is located in muscle cells), is produced in great quantities when oxygen is present (36 ATP through the Krebs Cycle). When we take our training to anaerobic levels during exercise, our body goes into oxygen debt- which causes the body to bypass Krebs Cycle and utilize the anaerobic energy system- anaerobic glycolysis. During this process, only two ATP are produced. The bypass happens in the pyruvate step of the Krebs Cycle- when oxygen is present, carbohydrates and fats are metabolized and broken down to enter the Krebs Cycle. The subsequent molecule produced, pyruvate, allows the Krebs Cycle to occur by binding to the hydrogen ions released from other chemical reactions in the mitochondria, producing the full 36 ATP. In oxygen debt, pyruvate is converted to lactate because when insufficient amounts of oxygen are available, the body uses carbohydrates (quick energy source) to break down into pyruvate and hydrogen ions. The excess hydrogen ions bind to pyruvate, creating lactic acid, which leaks into the bloodstream, giving that burning feeling in the muscle. The burning feeling isn't the lactate, really- it's the lactate binding to nerves in the muscles that create the sensation of burning. This is the onset of blood lactate accumulation (OBLA).
How to control this? For the most immediate answer, if you are out on your ride, you will have to slow down and take in more oxygen. Once you do that, the oxygen will drive that Krebs Cycle, lacate will convert back to pyruvate, and you're back in business, because you'll be producing more ATP- the energy used to move those muscles so you can ride that bike! That really is the primary way you stop the burn from happening. But there are a few things you should do in the future so you can ride harder and longer:
1) The most important thing you can do is base building. This really is done in the off season. Lots of mileage on the bike, spinning it out, and building your base is essential.
2) Coming out of your base, start working on changing up your rides- you need to increase your VO2 max through high intensity efforts. You need to do some tempo training rides to build your fitness level and get your body to tolerate these lactate threshold efforts. You need to do interval rides so you can learn how to recover quickly (among other things). And finally, you need to do some at/around lactate threshold rides- riding either at lactate threshold, slightly over it (about 5 beats over), and slightly under lacate threshold (about 5 beats under). Rides like this will increase your fitness levels and help you from building high levels of lactate in your muscles. Rides like this will also stimulate your mitochondria to grow in size, and believe me- you WANT your mitochondria to be huge. The bigger your mitochondria, the more efficient they are.
3) While in the off season, LIFT. You don't have to lift heavy, but you do need to work on increasing muscle mass. Why? Because you want MORE mitochondria. More mitochondria gives your body more chances to produce more ATP. You don't have to look like Conan the Barbarian, but a good conditioning program that gives you some additional muscle mass is beneficial. Given that we tend to lose a small percentage of muscle once we enter our 30s, you really will be ok to engage in a muscle building program.
4) Hydrate. Learn to hydrate well, and keep that good habit going forever- the oxygen in water is another oxygen source the body can use, so take advantage of that!
5) Get tested. I mean REALLY tested. If you live near a Lifetime Fitness, you can ask them to do a New Leaf test on you. The New Leaf will test (among other things) VO2 max and correlate it to heart rate. From there, you can start training properly around your heart rate ranges that give you the greatest improvements. If you don't live near a Lifetime Fitness, then just find a performance testing lab in your area (they are dotted all over the place). Also, try universities with physical education/kinesiology departments that may have the equipment to train. If you get lucky, you may even find a university that is looking for test subjects to run these performance tests, so you could maybe get tested for free.
I know this is a lot, but I really think it's important to know _why_ you have lactate buildup. If you know a little behind the science of it all, then it makes it a whole lot easier to figure out how to work with it and overcome the problem.
As a final note, I highly recommend some articles by Dr. Len Kravitz, who is a friend of mine and is sooooooo wonderful at explaining stuff like this. He can break things down so well. I sometimes ask him to "break it down to stupid for me", and he will explain things in such a way that things tons of sense. Here are some articles that are really great reads:
How to assess lactate threshold:
http://drlenkravitz.com/Articles/LTassessment.html
Talking about the lactic acid "burn":
http://drlenkravitz.com/Articles/lactatearticle.html
Different types of training you can do to train lactate threshold:
http://drlenkravitz.com/Articles/lactatethreshold.html
I trust Kravitz implicitly, I've read nearly every article he's written, and I've attended lectures he's given on nearly every article he's written since I first ran across him in the fitness arena in 1999. He's fantastic.
Good luck with your training.
Koffee