Frosted Walnut Layer Cake

We had a church meeting last night, and as I know that quite a few people like walnut cake, I thought I would make this. It has three layers, and is covered with frosting.
The cake turned out fine, but I had a problem with the frosting.
The recipe says to whisk the egg whites and sugar over hot water for 10 - 12 minutes, until it is thick. After about 7 mins, my mixture was thick, so I thought that was ok. Apparently not!
As I had obviously not whisked it for long enough, the frosting never really set, and also had quite a grainy texture to it.
It tasted ok, and it got eaten, but I knew it wasn't right. At least I will know what to do next time! I still need to make Devil's food cake, and this has frosting on it, so I may yet be able to redeem myself!
I always feel bad giving the recipe a score when I know I have done something wrong. If the icing was right, it would probably get an 8.0.