On a mower engine, there are a few things that typically cause high RPMs. I’ll try to list them in order of ease/cost. I assume when you start the mower it instantly revs up to maximum throttle and stays there until shut down. Because the issue started while mowing, it is not likely an issue that can be corrected with a carburetor screw. Some mowers don’t have any adjustments at the carburetor today. Off hand, this sounds like an issue with the governor whether it be the governor itself or the linkage/springs that allow the governor to do its work. If it sounds like it is running at max speed all the time, I would advise parking the mower until you can fix it or have it serviced to avoid catastrophic engine failure. I would recommend to only run it for brief periods of time until the issue is resolved.
1) Check for a vacuum leaks - examine the rubber vacuum lines thoroughly for cracks, breaks, splits, etc.
2) Check your throttle linkage and springs - follow your accelerator lever cable down to the engine. A picture may help here. Compare what you see with images of your exact engine online to be sure. There should be a bracket that the accelerator cable attaches to. This should move freely when working the accelerator lever. Make sure all brackets, links, and springs are intact, not stretched, bent, or broken.
3) Check your governor lever, linkage, and springs - A picture may help here as well. Compare what you see with an image of your exact engine online to be sure. This will look similar to the throttle linkage. Make sure all brackets, links, and springs are intact, not stretched, bent, or broken. (I had a gentleman give me a mower that was less than a year old because he tried everything and couldn’t get it to run. It turned out, the governor linkage bracket was located on the front of the engine and was bumped and bent slightly inward when he was mowing under a heavy bush. Long story short, I bent it out slightly, had my son start and keep it running, then I had to bend it out further in small increments until the engine sounded like it was running smoothly at idle)
4) If all else fails, there is a part inside the engine called a governor. This part has the sole job of keeping the RPMs under control which in turn provides optimal gas efficiency during idle and it also protects the engine from over-throttling. Basically, it creates a limit for maximum RPMs that keeps them within the normal operating range in order to prevent premature overheating, excessive gas usage, and an engine running too fast could more or less shatter to pieces after a short time. This is typically not an expensive part, but the labor to replace it will likely be expensive as you typically have to pull the engine and open the engine crank case. Note: Another reason why you shouldn’t run it in this condition, if it is the governor at fault here, is because the governor will typically break apart inside the engine which could cause irreversible damage if left go. There is a bit of a catch with this statement as you will need to run the mower while troubleshooting and performing potential fixes.
I hope this helps.