The only time I'd sugest using the same obstacle over and over would be the 3 low walls at barbarian agility course, and then only if I had lvl 35 agility looking for 40. I did that, needed no food, plus I never failed (not one time). The rest of the course you need food for. Even if you do fail, (the guides say you can), you won't fail enough to hurt you. The lack of food weight helps a lot too.