Corn can be found in most pelleted feeds- especially sweet as filler-- you can see if it has corn on the tag.
We were feeding life designs compet pellet feed mixed with cracked oats in the winter time and when we are working them hard- the compete is high in fat and good for horses that burn more callories than they take in with the avrage feed-- in winter we would add hot water and some honey and oil.. tasty!
The coconut oil is tasty too! You just use it like any other oils on top of the feed or you can mix it with your hands if you want-- we put about a cup per horse- all the fat goes straight to the topline.
30 minutes a day on the flat- hill work might work but collection work is more efficient -- one of our geldings is the type that 'naturally collects' and his topline is so nice! With they were all that way, lol.
I got a walker I do a lot of collection and backing work with and her topline is very muscled up- I don't even do any hill work with her and hardly ride her up the mountians or hills- it definitely is more effective than just flat work but collection and backing works nicer in my experience working for topline.
For your horse I would build up his stamina and muscle on the flat for 30 minutes a day then go from there.