firstly, if you dig a little deeper at more modern studies you'll realize that LT as a concept has been relegated to myth status.. that LT inflection point doesn't exist.. actually it can't exist or human exercise physiology would be completely different than what is actually observed.
secondly, you would want to do a "Threshold" interval at a higher intensity than "LT"
thirdly, threshold intervals should be minimum ~10mins
fourthly, as others have mentioned if you don't have a power meter, you'd be much better find a course that you can do your interval (10min minimum to 30mins) and use time rather than HR to gauge your effort HR is not that good of a tool for gauging intensity of threshold intervals.. the time lag is too long and it HR varies too much on outside influences.. heat, time of day, fatigue long or short term etc, etc.. you'd be much better off just taking a few sessions and setting splits for your self. try an effort if it's too fast, go a little slower next time etc.. 'til you find a speed/time that approx. work for you... try to use the same course all the time so you can gauge what you are doing.