A few different things to consider here from a physiological standpoint. Depending on where you get hit on the leg you will either be taking a shot to muscle, to one of several nerves or both. The nerves do not run exactly in the same place in every person so it can be difficult to target an EXACT spot on an opponent. From your description of numbness and loss of mobility you probably took a kick on a nerve. The nerve gets mashed between the opponents tibia and your own femur and mashing nerves never leads to anything good. Mashing muscle can typically be dealt with easier since the main factor to overcome (at least during the fight) is pain.
Now, on to conditioning. Just plain old taking a lot of leg kicks and slowly building the intensity of the kicks will over time have a dual effect. First you will get more and more used to the kicks and your tolerance will build due to the tissues becoming used to the repeated trauma. Second, even though the signal to the brain will still be telling you that you have a lot of pain your brain and body will learn to deal with the situation faster and you can dictate less of a physiological response. Look at it like this, if you stub your toe in the middle of the night it takes a few minutes to deal with the pain because you are not conditioned to it and it came as a suprise. However, if you have been training to take leg kicks and you see it coming your brain processes the pain signal and deals with it in a matter of seconds or less.
Now, as far as weight training, muscle density building goes this can have either a positive effect or a negative one. The nerves in our legs run through the muscle tissues. If you add more muscle you may either be adding tissue on top of the nerve and this will provide more cushioning againts a kick. OR, the muscle tissue may be built under the nerve and this in turn pushes it closer to the surface and makes it easier to injure the nerve with a kick. This is why some very muscular people seem immune to pressure point techniques while others seem almost overly sensitive. Unfortunately this aspect of either "burying" an nerve or pushing it to the surface is genetically predetermined and is out of our control.
While I agree that it is best to block a kick or move out of it's path I think that conditioning for getting hit is a VERY important aspect of training. I am reminded of a great quote from a teacher/coach of mine. "There has never been a world champion fighter that hasn't been hit."
Best in Health and Training, J. R.