I agree!!  You don't need to spend money on expensive classes etc… a good brisk (ish) walk can be very beneficial for you.  I guess you know yourself how tired you feel, and rest is very expensive.  I have a friend who managed to stay very active right up until the end of her pregnancy and she believed it really helped with the birth and her energy levels after. 

I have another friend, who at 41 has just had her second child though and she had quite a different experience.  She seemed to get very bad water retention at the end, and although she was a month early, she had very swollen legs and ankles and had to rest for the majority of her pregnancy.  Fortunately, mother and baby are now doing great!!!