It all depends on your level of fitness of course. Before I started I hadn't done anything more active than walking around town and to and from work.
I found that for the first couple of sessions my quads weren't up to it, all that seiza and kihon dosa practice, so for the next day or so I'd have trouble going up stairs!
But having said that, going to Aikido twice a week was all I needed to do to work those kinks out. Its depends a lot on your dojo and how intense the physical training is.
Just remember to warm up and warm down properly. If you're not sure how, look into it before you go. Its possible to go through all the exercises at the beginning of class and still be no more warmed up than if you'd just walked in off the street!
Welcome to Aikido, this could be the start of something beautiful!