I wouldn't start riding her everyday straightaway! Take it slowly.
Hmm rice pollard puts weight on, but be very carefull with that! As it puts fat around the organs.
I would suggest boiled barley, it puts safe weight on and its non heating. But its a pain cause you have to boil it!!! You can get steam rolled barley but its not as good.
Whats type of feed is in stratergy?
If you are going to eventually be riding her everyday, she will need something other then hay, something for energy.
What type of riding are you doing with her?