Cindy - add some weight training specifically upper body. I forget who said it, but youu don't play tennis to get in shape, you get in shape to play tennis. Especially as we age.
re: portable ball machine - The problem is that nobody wants to have enough balls to hit long enough, THEN they set the machine to hit balls to them too fast. Slow down the rate at whihc balls are hit at you so you can THINK about the stroke. It's PRACTICE time, NOT Aeorbics time.
Have a plan set up when you go to do a ball machine workout. Half the "fiddle with it" time is b/c you set it to throw you this ball and you change your mind 1/2 way through.
Finally, realize that when you feel like you didn't get a workout ro whatever, count how many balls you actually hit. You will be amazed. Depending on the size of the machine, yopu can hit 50 or 100 shots in a row before needing to stop and reload. If the machine is not programmable, you can have it send shots to a certain area. load 50 balls. hit 25 forehands, then shift to 25 backhands from the same spot. You will be amazed how little time it takes you to hit 300 balls!
Finally, ball machine practice can get boring and it is very easy to let your technique break down as you tire (and you will tire). I like to bring soem towels to the court with me as placement targets. They are big enough to be a realistic target and roll up in your bag very easily. then you can have a realisitc placement goal for your practice.