Homocysteine is a blood test, but not in the standard chem panels which are groupings normally used.

In a young person, it is not typically done either. But if it is elevated, it would point to a low B12 utilization factor, or a genetic problem where the body cannot convert/methylate B12 and folic acid. This is pretty common, and not rare. But not all doctors look for it either.

When you engage in extreme exercise you place yourself on an edge of sorts. Without the activity, you might present later in life with the same symptoms. WITH the activity, it may accelerate whatever your weaknesses are. They could be metabolic, (light flashes in eyes point to this), or mechanical, as in disc stress in the spine.

To get enough magnesium, you have to have 400mg a day and an example of foods to consume are 3oz of Almonds a day (270mg right there) and beans, oatmeal as examples.

Having a V8 vegetable juice after a run will provide 800mg of potassium. This is one of the highest sources. Energy drinks in comparison are very low, with Gatorade at 30mg.
An average person now is suggested, to consume 4.5 grams (4500mg) of potassium rich foods a day.

If your Vit D is low, there can be derangements in calcium and when this happens, you get paresthesias, or odd tingling/numbness. So getting Vit D tested can be helpful. Many young women have been found low in calcium and their bones are being robbed for it, today.
http://www.nof.org/aboutosteoporosis...oungadultwomen
I saw an ABC medical segment over 10 yrs ago on this subject and it is still a serious issue. If your running is impacting your periods, it is a greater issue even.
Your goal for Vit D today is 50, that is the newer suggestions.
If you don't have good Vit D status, then you are not absorbing calcium from foods. Don't assume that if you are eating well, all those nutrients are going where they are supposed to be going.

There are many factors that could be going on. A neurologist will be looking for organic factors, tissue/nerve damage only. Typically they don't address metabolic nutritional factors.

Looking at your lifestyle, the flashes in your eyes happen after a run, and to me they are the most important sign, unless you haven't told us everything. They can come from the retina itself, or the brain. If from the retina, that can be low blood pressure, spasms of the little vessels, high blood pressure clamping down on them, low blood sugar, or some autoimmune problem in the cells themselves. You might want to see a good opthamologist MD for this if it continues. If from the brain, I'd be concerned if you take birth control pills, as these can cause strokes in otherwise healthy young women.
Homocysteine elevations often show up in the vessels of the eye. And while you are young, you could still have this, if you are not methylating B12 and folic acid properly (this is genetic).

Reliance on fancy conduction tests, will only reveal damage to nerves and not all sensory damage even shows up on them.
So do consider these other issues, as well.