If it's a sudden and notable change, check your brake fluid level and look for signs of a leak around the brakes, the brake fluid reservoir, and the master cylinder. It could also be a mechanical problem somewhere, such as an adjustment issue on the rear drum brakes.
It's very common for the brake pedal to be soft from day one. Mine certainly has been - I was disappointed on the test drive. It obviously didn't stop me from buying the truck, but it bugs me that the pedal feel is really mushy. If you slam on the brakes, the brake booster kicks in and the feel is much different (better). All I can figure is that it's a combination of the front disc/rear drum setup (as opposed to being discs on all 4 wheels) and maybe cheap brake lines. I've seen posts about aftermarket brake lines helping to solve the problem. There is no actual problem with the brakes - they work fine - but the pedal feel is not good.
I had that problem occur on me once. Almost didn't stop in time. I don't know what caused it. I'd like to believe I just didn't pump the brakes enough as I came down the long hill, but it still bugs me to this day why it happened.