Lactate (lactic acid) is a byproduct produced by type II (fast twitch) muscle fibres when they process glucose for fuel. It is the substance that makes your muscles hurt when you are exercising.
The good news is that your type I (slow twitch) muscle fibres reprocess the lactate back into glucose for reuse by the type II muscle fibres.
All is well whilst the type I muscle fibres have the capacity to reprocess the lactate being produced by the type II muscle fibres. But if you exercise at a level of intensity that means the type II muscle fibres are producing more lactate than the type I fibres can cope with, things start to go wrong. Any excess lactate must be passed into the blood stream to be processed elsewhere in the body and if you continue to maintain the same level of activity more and more lactate is produced and the amount in the blood rises exponentially until the body blows up. Not literally but we all know that feeling when we suddenly cannot carry on pushing anymore and have to rapidly drop our level of effort considerably.
Your lactate threshold is the point at which the lactate in your blood begins to rise exponentially. On the road it represents the hardest you can ride for a prolonged period of time without blowing up.
Where to next?
Back to interval training for cycling Lands End to John O’Groats, look at endurance training or visit one the most popular pages on the site, which concern planning your End to End, thinking about the cycling equipment you will need, what you should be eating and drinking whilst cycling and how to create a route for Lands End to John O Groats. Or you can read my own account of cycling End to End to get some idea of what to expect.