I played football with guys that were dizzy after every practice. I bet your issue is most likely due to temperature, being out of shape, and a hyped up sympathetic nervous system (anxiety related). Take a step back and try walking walking during a time of day that is not extremely hot. Also I am a huge health nut and can tell you diet is 75% of the battle. Exercise only takes you that additional step forward to help in losing weight. Best advice for weight loss is cut out simple carbs (potatoes, fries, rice, bread). I live my life around eating fruits, vegetables, and lean meats. I don't drink pop, alcohol, or juices.
I was 260lbs and lost 80lbs mostly by diet and exercise combo. I also was borderline diabetic and near to being placed on high blood pressure medication. Currently my blood pressure is in the average of 115/62 with a pulse rate of 62. My LDL is also near non-existent and furthest away from being diabetic. Trust me if you want to lose weight it is achievable. If you need anymore weight loss advice please message me. I help a lot of people lose weight and change their life around.