You should be OK, especially riding around on bascially flat ground. Two strokes are very unforgiving, but the flat ground will make up for that.
I suspect that before long you will wish you had a 250. The 125 will be great for starting off but I wouldn't invest too much in your first bike. I certainly wouldn't go anthing smaller than a 125 with your height and weight.
Get used to the idea of your ass being slammed to the ground, it is going to happen. Get the proper protective gear: boots, helmet, goggles and chest protector. I also highly recommend knee pads and elbow pads. The riding pants may seem like an unnecessary expense but they will survive a lot more abuse than a pair of Levis will, and after you've torn a few good pair of levis you would have paid for the riding pants.
Rod