Actually, it doesn't have to be fast. The key depends on persistence. I think it's better to start with a diet, otherwise it will be difficult to have an effect. It also needs heavy load and reasonable training. It's best to hire a personal trainer. It doesn't matter if you don't. Personal trainers only play a supervisory role. If you have perseverance, you don't have to think of any good methods. Just running can be reduced or persisted.