My two cents. Max HR is the fastest your heart can beat. This definition means that you will rarely, if ever, see this value. People who do approach it or reach it report tunnel vision, visual spotting, blackouts, nausea, vomiting and the feeling that death is preferable. In a nutshell, the finishing sprint in a crit.
Your LTHR is a maximum "sustainable" HR, and of course sustainable does not mean for 4 minutes. The definition in this case will be the HR you could ride at for an hour assuming normal drift and averaging. You don't obtain this value by sprinting as hard as you can and then going as fast as you can until you topple over. you assume that you are going to be riding for an hour and you pace yourself as best as you are able. you will find that your legs will tell you when you are at LT. If you maintained a steady pace for the entire hour, your HR would start low, and slowly climb, crossing your LTHR until it would be a few beats over that in the end. However, the average would pretty much zero in on your LTHR.
Should you not want to ride for an hour "all out" then a more feasible thing to do is to ride a 30 minute TT (as Friel suggests) and take the average HR of the last 20 minutes. This will bring you within a beat or two quite handily.
Is it possible to run at 195bpm for an hour? You are young, possibly untrained (lower stroke volume thus higher HR), and with HR variability, I would say it is possible to be 195. Having said that, I doubt it. It would certainly be among the outliers.
Say you understand now.