Soft braking that doesn't stop well, what could it be?

Don't let it stay like this, that's a serious safety risk. Don't try to keep driving long distances with that problem, especially not on the highway. Air in the lines is a real possibility and pretty common, especially if it's been a while since you last serviced your braking system. When air gets trapped, the fluid doesn't transmit pressure correctly and your pedal becomes spongy, without firm response.

To figure out if it's air, the first thing is to bleed the brake lines. It's work you can do yourself if you have some patience, or you can take it to a mechanic - it's not complicated or expensive. You'll need a helper, a container, a hose, and the correct brake fluid for your model. You open the bleed valves on each wheel (usually starting with the one furthest from the master cylinder), press the pedal several times until fluid comes out without any bubbles. If the brake is still soft after bleeding, then the problem is something else.

Other possible causes: the master cylinder might be losing pressure internally (those seals wear out), the pads could be almost at their limit with less material left, or there could be a small leak in some hose that you're not noticing easily. Also check if the pedal sinks on its own with the engine off after braking - that indicates a leak. Either way, don't wait long to get this checked out properly. Braking safety isn't something you put off.