Well I had a P0170 code also. I was hoping the LTFT might help to confirm a bad MAF diagnosis. I checked the fuel pressure regulator and it was good. I inspected the vacuum lines. So I put in a new MAF. The LTFT has been falling ever since. The reading has changed like this:

Before changing MAF 31
After 9 mile trip 28
Another 9 mile trip 25
6 mile trip 22
6 mile trip 21
30 miles 18
30 miles 17 current reading.

So I guess I high LTFT reading could indicate an aging MAF. At least it seems that way in this case.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra