Mower deck Belt won't stay on

I have a model 13AX795004 42" MTD Gold Hydro. I replaced the blades last weekend, cut about half of the yard (2 of 4 acres) and the belt came off. The grass is high due to a lot of rain and a tight work schedule. I replaced the belt with a 90" belt and I'm still having the same issue of the belt coming off. Any suggestions would be appreciated. Oh, and I would take it to a mower shop, but I don't have a way to transport it.