You definitely would not be stupid to join! I joined a beginners' running group when I first started running. It was brilliant and very motivating. Hopefully you will find, like I did at my group, that the instructor will find ways of catering to everyone, regardless of their fitness level. At mine there were some quite fit people and some reeeally slow people who found it hard at first. Everyone was very supportive and the slower ones soon got a bit quicker. Well done you for being up for the challenge! As far as shoes are concerned, you should really have ones meant for running, as they need to be able to take a fair old pounding. If yours are ancient or not for running then they may not be supportive enough. A good sports bra is essential too, particularly for those of large norkage! Good luck!