I've never found walking to do anything for my cardiovascular health. I wouldn't worry about adding a couple of running/biking sessions a week, they'll help a lot more and the interference with your lifting will be minimal.
Also, if you're planning on comepeting in strongman (I hope I have the right guy here), you're gonna need more endurance than you'll get through walking.
I like sprinting/interval workouts myself, and I've never had any real interference with my lifting.
Try out 1-3 harder sessions a week, experiment and see what works best for you.
Good luck
cuthbert