Yes, I think even 15 minutes a day will have a good effect. I started very gradually, but within a couple of months I could walk 12 kms on very steep paths (the only kind they have round here!) without getting out of breath. Like most converts, I am incredibly passionate about the advantages of walking.
I have walked for two hours this morning, in the heat, carrying a backpack which probably weighs about 2 kgs. (Less once I had eaten my lunch and drunk most of my water). I climbed about 200 metres, over 600' , over very rough terrain, including fallen trees and branches and through some very deep muddy patches, which made me grateful for my sturdy, waterproof boots and my walking poles for balance.
I would have walked for longer, but the men's final called! I might go another walk this evening.