I have three horses, ages 16, 17, and 30. None of them are currently on senior feed. However, I do feed alfalfa and grass hay. In the summer, when I ride the teenagers (the 30yr old is retired) I give them about 1- 2 pounds of sweet feed, once a day. In the winter, when I can't ride as much, and don't want them to get too much energy, I usually feed something like a couple pounds of alfalfa pellets with a little vegetable oil drizzled over the top. Maybe a little rice bran in there too. I like to soak them a bucket of pellets and make them a warm mash when the weather is cold.
Strategy, in my opinion, is a great feed too. The only thing is, around here the Strategy costs almost as much as the senior feed, so if you like giving her senior feed there isn't much difference price-wise between that and Strategy. They say beet pulp is also very good for keeping weight on a horse, but I have never fed it myself.
I never considered a 12-13yr old a senior. I really don't call them seniors until they hit 20.
PS. You should be able to find both grain and non-grain feeds at your local feed dealer.