A slipping odometer, a very common problem, will reduce the "appearant" milage considerably! Check the odo against some interstate milage markers before panic!
As to the gears, the TE starts in 2nd unless you floor it and hold the kickdown switch closed, or start off in S or L rather than Drive. Give that a try before thinking you aren't in top gear.
You will be pulling 3300 to 3500 rpm at 70 mph, I think, or something at least higher than 3000. Pretty low gearing.
Actual highway milage should be in the low 20s, mine is. 19 in town
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I would also check the mixture, or at least teh tailpipe. If you have black soot around the end of the pipe, you are running rich and need to get that fixed.
Peter