If you have to watch SD, you have to move back. In analog days, there was a reason for the old rule of viewing distance = 3 x diagonal. The reason is resolution, and SD basically duplicates analog, maybe slightly better or worse, but basically.
HD has more resolution and viewing distance rules are more like 1.5 x diagonal. If you view SD at that distance, it will absolutely suck. Move back to 2.5 or 3 X diagonal, it will look like SD. You will still be spoiled by HD and not want to watch SD, but it won't be COMPLETELY awful.