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Any change to color or temperature of legs/feet?
What have you been doing within the last week? Any unusual activities?
Where is the pain located? Does it move?
Does anything make it better or worse?
With sudden onset pain in both legs then I would be worried about reduced circulation, but you do not mention any change in color temperature that would support this. Also your age makes this a lower risk.
Other possible causes include trapped nerve although the fact that it is in both thighs makes this less likely although not impossible.
As the advil and heat give you some relief then that suggests there is inflammation or muscle spasm occurring. In an older person I might suggest a problem like polymyalgia rheumatica.
Given the severity of the pain then I would suggest that you go to ER for assessment of circulation and musculoskeletal examination. They will be able to give you stronger anti-inflammatory that may help, as well as something for muscle spasm if this is deemed appropriate. They will also be able to do blood test to look for raised inflammatory markers.