Replacing running with fast-walking?

Other than the number of calories burned, is there any *serious* down side to replacing running with fast walking (at approx. 5-9 km per hour)?

I’m starting FB Low Impact for the second time tomorrow and want to replace the running part with fast walking in the PFT. I did it in the PFT at the end of the program and somehow actually had better results than the beginning, in which I ran and felt terrible and nearly slid off the treadmill.

I have no specific health condition, just not very fit generally, and I don’t need to lose weight (58kg-ish, 155cm) so I don’t mind burning fewer calories.