Lack of progress using elliptical trainer

I am 61 and in reasonbly good condition - trying to improve through
regular exercise with my dr's concurrence.

In addition to regular workouts on my Total Gym I spend 45 minutes
daily on my elliptical trainer (Precor EFX) using variable resistance
and pedaling backwards about half the time.

I have been following this regimen faithfully for 6 months, yet I see
no evidence of improved leg function. When I walk my legs feel stiff
and sore, and I seem to have no more stamina now than I did 6 months
ago. I tire just as easily and become short of breath just as quickly
as before.

Can anybody suggest why I don't seem to be seeing signs of improvement?

Thanks
Dave