Can I gain cardio fitness by walking?

Background is that I've had a lot of knee and ankle injuries, and I've been told running is a really bad idea, because I'll hurt myself again, and it's too high impact.
I really want to gain some cardio fitness, though. If I do a lot of power walking, will that help me? I can't afford a gym membership, they're very expensive here.
If walking won't help, can anyone recommend another way?
Can I gain cardio fitness by walking?
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