text: First of all, you can't exercise "too long". It depends on what your objectives are. If you stay longer than 40 mins on the treadmill, you're losing more fat and building up more endurance. If you stay on for 30-45 mins, that's a good time for people who just want to lose weight and keep in shape. After 30 mins, you start burning fat instead of just calories. So the longer you stay on, the more fat you're burning. People who train for marathons and races definitely stay longer than 45 mins! \nBut in your case, I think 30-45 mins is a great time to start off with. And don't worry, my face also gets extremely flushed after working out, this has nothing to do with your level of fitness.\nYou will see that the longer you stick with it, the better your overall fitness will become, and eventually you can start jogging. I can also recommend that you perhaps try the uphill mode on the treadmill - it'll burn more calories and tone your butt as well. So, I think you should keep up the brisk walk for now, and start jogging whenever you're ready for it (and you will be). However, if you really don't like jogging, just keep walking. It definitely puts less stress on your knees and as long as you're exercising, there's nothing wrong with walking! The key is to work up a sweat and get your heart rate up.