My advice is to just continue as you are. You have good fitness already but need to build up 'miles in your legs' before doing speed work.
I'll repeat myself and say you really should find a half marathon to run. This will be a valuable learning experience before the big day. It'll teach you about starting out to quickly and it will also give you proper paces for training.
As for Lactate Threshold: You store energy (glycogen) in your blood stream, liver and muscles. As you exercise your muscles use up the energy and produce waste (lactate). Your blood carries this back to the liver where it is converted back to energy (glycogen) and the cycle repeats. At low intensities the lactate gets converted very quickly. As the intensity increases there comes a point where you're using glycogen and producing lactate quicker than your liver can process it. This is your Lactate Threshold. By training at that level your body adapts and becomes more efficient at doing the conversion.
All very technical. Your LT pace is somewhere between your 10k and your Half Marathon pace. I believe running around 20mins a week at that pace should be enough.
More reason for you to do a half and determine a proper pace for your training.
It's more about specifically targeting your training to cover your weaknesses.
It's quite easy to just go out and plod away slowly for several hours on a Sunday believing that it's doing us good.
There are a lot of training plans and a wealth of wisdom out there.
I'm pretty much sold on the long run not being much longer than 2:30. I'm stretching it to 2:40. The intervals at VO2 max to raise my base fitness and the threshold runs to work on the lactate threshold.
Last year I worked hard on my LT runs and saw almost immediate improvements, dropping 5mins off my half in only a few weeks.
The long run helped me continue running through 18miles on the day having started out at a pace slightly faster than my long run training pace. But getting stressed over trying to fit multiple 20milers in that were taking 3:20 and leaving me drained for two or three days after aren't something I'm keen to repeat. Having done 4 marathons now, I may possibly do one 20mile run but think it's unlikely unless I suddenly gain masses of speed.