Can I lose weight by dieting alone?

Yes, you definitely can. I did. It was easier for me to start exercising more regularly after I lost some of the weight. I think it's healthier to exercise,but I always am hungrier when I exercise so I eat more. To just lose weight quickly (about 50 pounds over about 7-8 months), eating at a deficit worked for me. I used to think you could lose weight by adding exercise. For me, it was all about calorie control. Having said that, I feel much better physically and emotionally when I get regular exercise. I sleep better and just feel better overall.