2001 Ford Truck Lack of take off power and poor gas milage

I have a loss of power mostly on the take off. I have owned this truck from day one and I have taken very good care of it. I just pumped the Trans. and put a new filter in. Oil chang every 3000 miles with filter. Just changed fuil filter. That did help but not right yet. I have read about a idle air control valve that should be cleaned. I don't know where it is or the best way to clean it . Also had ford change my plugs @85000 miles. She just is slow to reach driveing speeds. I am not a hot rod kind of person but I do run with the traffic. I seem to have to give it more gas than I use to to reach these speeds. My MPG have drooped also. I check every tank so I know how she is doing. In the last year slowly she has gone from 17.4 MPG to 12.85 MPG I have used Sea Fome to clean up the fule system. Any help would be nice . She starts and runs very nice and I would go any where in her and not worry . Do you think it's just the miles on her that hads knocked down my MPG ? Thanks Oh yes she is a 2001 f150 5.4 2 wheel dr. Auto.