Welcome to NeuroTalk:

You will want to double check with your doctor, for the exact level of B12 you tested at. Lab ranges in US are low, and if you are below the new standard cut off of 400, you could have NOT normal results. Some doctors don't know this yet.

That feeling of water going down the legs, is also present when there is compression in the spine somewhere or elsewhere. I had that symptom for my MP (meralgia paresthetica), before it became very painful. MP is a compression of the lateral femoral sensory nerve. Mine was from a C-section surgery. MP can come from wearing too tight jeans, or from surgery or trauma.

Please read our subforum above, and see if anything is familiar to you. Many PNs are environmental, meaning something you are eating or not eating, or exposed to (toxins, drugs, vaccines) can be triggering it. There are hereditary PNs...
http://neurotalk.psychcentral.com/thread121564.html

Flashes in vision, suggest a circulation problem, or low blood sugar. Sometimes they are auras of a migraine. Flashes in vision without the headache are called silent migraines.
Did you have a homocysteine test? When elevated this can affect the blood vessels and damage them. If the flashes are more towards the end of your run, it is the stress somehow from the run I'd look at.
Also you may be low in potassium and magnesium, which create weakness and circulation problems. About 70% of Americans do not eat foods with magnesium in them. So they walk around in a partially deficient state, and then add to that vigorous exercise and you have an electrolyte problem.

Sometimes you have to be your own detective to find your trigger. Since you are heavily into running, I'd suggest you stop for a month or two and see what develops or changes.