Have you considered anything that might stress your joints a little less than walking? I know it's probably the cheapest form of exercise, but I have a tendency to waddle that annoys me to no end. I've found that if I'm going out with the sole purpose of exercising (and enjoying the outdoors in general), I prefer to take my bike.
I'm lucky (and I know it) because my husbeast realized that there was a problem when he'd be half a km ahead of me on his (fixed gear) bike, and I'd be chugging away, turning red as a raspberry on my poor little trail bike. This is with him "going slow"**. You see, he bike commutes to work, while I take the car. It's one of the weird benefits of him not having a driver's license!
He's also one of those weird "eat less and move more" people. It boggles him that I lose weight when I stop eating hot dogs, hamburgers (the buns in both), pasta, rice and potatoes.
The solution? we saved our money and bought a couple of sturdy 5-gear city bikes. I'm still slower than him, but not by much. It's also easier for him to hold back a bit due to the style of bike. And he knows that if I'm walking up the hill, it's steep enough to get the best of me. Our city bikes are much heavier (look up Pashley on Google. His is a roadster sovereign, mine a princess sovereign), so he has a bit more challenge, even though he isn't going as fast. It puts us on a slightly more equal footing!
**For those who don't know what a fixed gear bike is, well...take all the gears and brakes off your bike, as well as the freewheel (thing that allows you to pedal backwards with that 'click-click-click' noise I thought was so cool at age 12). That's a fixie. They're really REALLY light, usually made from old competitive racing bikes or purposely built for fixed gear speed. You don't stop unless you slow your legs. Your legs *are* the brakes. Fixie riders do a kind of skid to try and slow themselves down, because once they get going, it's difficult to stop. Also, he's been known to hit speeds around 40 or 50kph when he really gets going in traffic, so slowing down for me is a bit of a pain.
(btw, he's about 250lbs -- easily 40 pounds lighter than me, and has been cycling for a couple of years, thus having more muscle. He definitely has an advantage).