If you don't own an HRM, then you can estimate it rather easily. If you typically run a 10K in one hour, then your LTHR pace is approximately 10 minutes per mile. If you're still unsure, then you can always monitor your breathing. Lactate threshold pace corresponds rather nicely with your ventilatory threshold. When your running pace gets fast enough that your breathing becomes laboured and it is difficult to carry on a conversation, you are running very close to your LTHR pace.
Breathing rate becomes a very useful tool when training on the trails.
Lactate threshold improvement comes from doing longer intervals of up to 30 minutes at your LTHR pace. For the more experienced racer, two to three sessions per week is definitely worth aiming for. Just be careful to monitor your body for signs of overtraining and fatigue. Everyone handles recovery from hard sessions a little differently. After six to eight weeks of LTHR training, you will be ready to move on to shorter intervals and perhaps speedwork on the track.