LTHR is simply the highest heart rate you can sustain (read average) for an hour, in a quasi-steady state effort. However it’s difficult to maintain concentration/motivation for an hour so instead you’re probably best off using the ‘friel method, found here: http://www.trainingbible.com/joesblog/2009/11/quick-guide-to-setting-zones.html
I’d be wary of training by heart rate given how inconsistent it is, caffeine, hydration and heat, among other things can have a huge effect – I’d say you’re far better of using RPE or Power. For instance a good dose of caffeine means I can hold 200bpm for a fair while, without caffeine I max at 201bpm.
And max heart rate means very little since it’s quite an arbitrary measure, in that it can yield different relative intensities to different people. So whilst your zones based upon MHR may say you’re in Z3, in terms of output you may be z2, this may be because you can sustain a relatively high heart rate compared to the average person. So yeah, whatever the highest heart rate you have ever seen on your HRM, that’s going to close to your MHR – but you don’t really need to know.
Edit: Technically LT is the wattage at the point at which lactic acid begins to accumulate within the body (A few labs use your baseline lactic level +1), but that’s another story.