Spasso said:
A mushy pedal is usually a dead give away that the brakes need bleeding due to air in the lines OR the the master cylinder piston seals are toast and the fluid is 'blowing by' the piston instead of creating pressure.
An easy way to check if it is air in the brake system is to (in the driveway or garage) press the brake pedal down, release it and then press it again. Is it higher the second time? Try pumping the pedal. Does the pedal get higher? This compresses the air in the system and all you would need is a simple brake bleed. To tell if a piston is leaking by, press the brake pedal hard and hold it. If a seal is leaking by the pedal will slowly sink. This is a problem that requires immediate attention like Spasso suggests. If it is master cylinders seals, it leaks internally and you would not be able to see it. For a caliper seal, it will leak by the dust boot and will notice a consumption of brake fluid and probably evidence of fliud leakage on the inboard side of the tire.
For a general soft pedal it can be just contaminated brake fluid, but I would take a good look at the hoses like Steven suggests. If the hoses have become soft due to being oil soaked or just from age they can swell when you step on the brake. Not all your pedal pressure is getting to the calipers, some of it is being used to swell the hoses. The system doesn't have a large capacity so it would not take much hose swell to be felt in the pedal.
At any rate, the ability to stop is more important that the ability to go. Where you have enough concern that you double your following distance for fear of not being able to stop, it is time to get them fixed without delay.