My father has walked for years and has seen the benefits. He really moves (his pace is faster than some people jog). No leisurely stroll. By moving and getting your heart rate up, you will get the maximum benefit (calories and cardio). You may want to check out a heart rate monitor. I know several runners/walkers that swear by them. good luck.
Welcome! The fact that you are not afraid to spar higher ranks will accelerate your learning curve, because by working with these people you will learn the most.
One thing, the horses really make it hard to keep the dojo clean! Oh my, no disrespect, but I think kenpo somehow got mixed up with summer camp. For accurate information on kenpo/kempo try... http://www.urbin.net/EWW/MA/KEMPO/kempo.html
Like Delta1, I think Kempo may be a good choice. Along with the adaptability, some schools also incorporate some classical material into their forms and katas, along with grappling and self defence.
To have good ab definition you need low body fat. If you are looking to reduce excess around the waist then you need to improve your diet and incorporate the cardio into your routine.
Try the kempo school and see if they do the groundwork. My kempo school does grappling and jui-jitsu techniques. I found it to be a very balanced mix with the striking/kicking/kempos/self defense. The best advice is to sit in a few classes in each school and observe. Good luck.