Can you build an aerobic base with walking?
If you've been severely deconditioned/sedentary, extremely ill, and/or bedridden for a long time then yes, absolutely.
Rocky Marciano was a cardio machine and on top of all of his normal roadwork, boxing training, and strength/conditioning he was known to take very long walks in the evening after dinner. Kinda hard to say if that was the secret to his success. The terrain and speed at which you walk will make a difference too.