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I found I have the same kind of experience here.
a couple of years ago I could barely sit for 5 minutes without going numb, these days I sit for about 25 minutes before my legs go numb, (I do have to lay around unable to walk for a couple of minutes afterwards though)
Having said that I still have days when i cannot sit for more than 10-15 minutes, it depends on lots of factors i suppose, but in general there is a trend towards progression, albeit very slowly.
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