Re: New To Martial Arts - Aikikai Aikido
Terry, welcome to aikiweb.
Many folks have taken up aikido later in life and the fact that you come to it with a higher-than-average fitness level certainly will help. Making sure you are warmed up before each class and pacing yourself will always help. If you aren't doing any kind of flexibility or stretching exercise, adding that in in advance of starting may help too.
Thanks for your service - safe return home!