You should concentrate on the weights my man. Walking is what you do to get from one place to another. Not an exercise.
Hit the gym, start out within your capabilities and work your way up 'til you are groaning with effort and think you are not going to make it. When you get that feeling that's when you know you HAVE made it.
Work hard, don't worry about your heart rate.
You will lose much more fat by lifting weights than by walking or even jogging. It's becaue you build muscle and raise your overall metabolism 24 hours a day rather than just the 30 minutes you jog. Also post workout excess oxygen consumption is higher and lasts longer after weight training.
Of course, see your doctor before starting an exercise regimen, yadda yadda yadda.
Oh yeah your workouts should be based around the holy trinity: squats, bench press, and deadlifts.
JC