Well, you've come to the right place for advise...

OK, my advise isn't always good, but you may get a chuckle at some point. For the intent of the post I will be a serious as possible.
Several of the ideas already posted are good ones, so it's best to combine everything into one big ball and run with it. Joemoto is on the money, walking is best thing for you, so you must push for a metal detector in order to get into shape. Go with something used and inexpensive and tell hubby that you have a game to play.. The game is to see how long it will take before your finds cover the cost of a MD
Next... It seems your husband has already made a commitment to get the detector, so start leaving subtle hints around on "legal sized" paper ( I prefer the 11" x 17" canary yellow myself ) and pin them at various spots hubby will be sure to pass. Leave ads, or print them regarding any MD right on the dinner table with little smiley faces on them too.
Last but not least... Again express your interest in both MD' ing and losing weight. Make a vow to set a goal for yourself and husband as to your weight loss plan and keep a progress chart.