Large diameter pipe will give you almost no heat gain (unless you cover your entire lot with it). So if you go that way with 50 feet, you might get one or two degrees.
What you need is a manifold setup that divides your large inlet line into numerous smaller lines (same as a store bought panel).
The reason you have to do a manifold is to keep the pressure relatively even, forcing the water into one small line only would create a ton of backpressure and wreck your pump pretty quickly.

To retain heat, put a solar blanket on it over night, it will cut down on your incurred looses greatly.