How much exercise?

Can anyone help me on this one?

I’m wondering how much exercise is too much? I did an exercise DVD this morning, thinking I wouldn’t have a chance to get out on the treadmill – but then my other half arrived home at lunchtime and said sure go on out on the treadmill, which I did for my 30 minutes. And now this evening I actually would have a chance to get back out on it again, I have the time and the energy. But I’m thinking that might be overdoing it. Would it?

I’ve been using Dr. Google and coming up with all sorts of different information. I’ve decided I will leave it for tonight because I haven’t done an exercise DVD for about six years, and I’m worried I’m going to be sore tomorrow anyway so I won’t push my luck! I’m just wondering going forward – is exercising twice a day too much?