So you have to start somewhere. The best way to know if you are fit enough for an Ashtanga class is to try one. If you have a relatively good overall level of fitness then you will be OK to join a general class. We practice to get and stay flexible and strong, not because we already are. In any yoga pose, you try to find your 'edge', that is the place or level in that posture that you can achieve without overdoing it but are still challenging your abilities. With regular practice the edge moves forward, we get slightly stronger, more flexible and balanced each time we practice. 

If you are totally out of shape then whatever exercise routine you choose to try will be a challenge, but you have to start somewhere.