Besides Greg Jennings suggestion about aerobic exercise - Nice Call Greg
, I would suggest for anyone - whatever their age to be careful about swari-waza
sometimes called the samurai walk
. This is practice walking on your knees. One of the first things to go on athletes are their knees, and moving on them doesn't help.
The dojo I go to doesn't do that much swari-waza or kneeling techniques (zagi). If your dojo does, go out and buy decent knee pads
. They even sell some especially made for Aikido. Wear them all the time, if you know you'll be doing kneeling techniques.
There are people who will say that they don't have any problems with their knees, but they're not in their 70's or 80's. The injury occurs over time and is not immediately felt.
Besides this one warning, just go out and have fun.
You'll find that you need to learn at your own pace. Sometimes that will be slower than someone younger than you. Sometimes that will be faster than someone younger than you.