Well James, D: drive is typically used for the Recovery portion or partition and should not be used for anything else and also typically it is nealy "full"or 95% should be about right. I suspect that it has gone on that way and just recently was noticed. It is nothing to be concerned about as this is normal. C: drive however may be the one with about 20% usage.
And there is nothing wrong with "I am the wrong side of 70". you are not alone.