Achy, tingly legs

[Scot__0__0]

  Hello,
  I began training for a marathon 9 weeks ago (37, male).  After 3 weeks of
  high intensity running both of my legs began to ache while sitting.  They
  feel tingly and crampy, and as though they are not getting enough blood.
  They feel weak until I walk for a few minutes then they feel better. My
  feet also ache once in a while, and both at the same time. Sitting seems
  to cause a fatigued feeling in my butt muscles.  After 10-15 days the
  symptoms subside, then return upon additional running (cycling seems to
  be OK).  Today I visited my sports medicine doctor.  Because of the
  bilateral symptoms he ruled out muscular overuse. An x-ray of the lower
  back appeared normal, and sciatica tests were OK. Reflexes of feet and
  legs seem normal, as are leg pulses.  He seems as mystified as I am, and
  suggested I may want to see my family doc for further ideas.  Any
  ideas for causes (or recommended tests)are appreciated!  
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Dear Scott,
Your symptoms aren't classic for any particular syndrome of the spine.  One might conder a central disc herniation in the lumbar spine, but again
the story isn't classic.  If your problems continue you should see a neurologist for opinion and complete evaluation.  He or she may want to get
an MRI to rule out any fixable lesions.  Short of that, a less strenuous
activity regimen may be all that is necessary.
Good luck.