Certainly all the rowing and walking I do has never helped me build muscle (it did help me lose weight though, not that I had a great deal to lose), building muscle didn't happen until I started to lift weights, and increase the amount of weight I was lifting incrementally, and I didn't need any fat to convert to achieve that.
No, you also needed protein to build muscle. You probably increased
your caloric intake to do this.
I didn't say I didn't use protein, of course I have protein shakes, and eat a lot of chicken and tuna and things like that. The point being though, that it is the process of damaging and repairing your muscles that helps them build, and fat is not a part of that equation.
This just sounds like an excuse for laziness, 791 calories is plenty to lose in an hours exercise. If you do that 5 times a week you've just wiped off two full days of calorie intake in a sensible diet.
If that's not enough to make a difference then you very possibly could be eating far too much, and not eating healthy food.