Subaru Repair: 2000 Subaru Outback auto trans, subaru outback, forward gears

Question
I just changed the fluid and filter (Dextron III), made sure I filled to the line, and took for a test drive.  That went fine.  When I tried to take it out again, it wouldn't go into drive.  The engine just races when I step on the gas in drive or any of the forward gears.  Reverse is OK.  Before I changed the fluid, I had a 2 to 3 second hesitation when shifting into drive, otherwise it worked fine.  I'm sure it has the right amount of fluid.  Any ideas?

Answer
Are you checking the fluid level on a flat surface?with the vehicle running in park?If you are doing that properly i am assuming you changed the external filter with an O.E. part and didn't take off the pan and mess with the screen.on a typical drain and fill, if done in a reasonable time you will change 4 to 5 qts ,so that about how much you would of needed to put in.Hope that helps.