Is going for a long walk twice a week enough exercise?

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Enough for what? You failed to explain what your fitness goals are so we can't tell you what "enough" exercise is. Walking 2x a week is not much, but it's better than nothing. You also didn't share what a "long" walk is for you, so again we don't have enough information to advise  you.

Depends how long a long walk is. 

I walk my dog anywhere from 30 mins to 1 hour per day, every single day. Now it's not fast enough for my heart rate to go up since my dog wants to sniff everything fairly often. I don't think it is really enough exercise. I've been contemplating whether I should start jogging. Just walking isn't going to compensate for not going to a gym. You want to walk or run fast enough to get your heart rate up. 

maybe 3 times a week

It depends on the distance. As per the recommended 10,000 steps that would be around 4 to 5 miles. Doing it twice a week is still better than not exercising at all, I admire your dedication to being fit.