Pilot reported inability to control engine RPM in flight and experienced engine roughness. Pilot returned to departure airport and landed.

Narrative

I was flying in a complex plane for the first time after earning my complex endorsement. I did a complete pre-flight and didn't notice anything amiss. However; when I did the idle check during run-up; it did act a little unusual. I attributed it at the time to the cold weather; but in hindsight; it was probably related to the problem I experienced. Specifically; when I pulled the throttle for the idle check; the power did not drop smoothly - audibly; it cut smoothly to maybe 25% and then sputtered a bit; then it dropped all the way down to idle. It did smooth out at idle though. I was still able to successfully increase power back to 1;000 RPM; and while taxing to the runway; I successfully increased and decreased throttle with no issue. I departed southeast from ZZZ; headed around south of the city to reach a lake; with the goal of flying into ZZZ1 along the river. I configured the engine to 25 squared successfully enroute. After getting to the lake; which is the southern end of a noise abatement corridor into ZZZ1; I turned north and decreased throttle to descend below the Bravo shelf. However; I began to pick up speed. I noticed that the engine was now running at 24 inches with 2;500 RPM. I pulled the throttle back to what should have been idle; but engine power did not change. At this point; I turned east to avoid running into ZZZ1 or ZZZ2 airspace while I diagnosed the problem. I pushed the prop back up to full forward so as to play with the throttle a bit. I was able to push the throttle up to max successfully; but I could not bring it below 23-24 inches. Each time I went to max throttle and back down again; it felt like it settled at slightly different pressures between 22 and 24 inches; but I am not sure if I imagined that. I tried cycling the throttle up and down a few times; but the bottom remained stubbornly high. At this point; I turned westbound to head back to ZZZ. At the lowest settings I could manage; the lowest speed I could get to in cruise; I was maintaining about 133 KIAS. So as to get below max gear extension speed (130 KIAS); I entered a brief climb; and dropped the gear when I hit 130. Gear down and a slight climb then brought the speed just low enough to barely get into the white arc; so I progressively put in full flaps. With gear down; zero throttle; and full flaps; I was maintaining altitude at around 90 KIAS; which is 20 KIAS over best approach speed on descent; so I knew this was going to be a fast and floaty landing at best. At this point; I was 15 NM southeast of ZZZ; so I called and [requested priority handling] from ZZZ Tower. They cleared me to land XXL; so I did a very long shallow approach so as to avoid picking up speed. About 50 ft. from the threshold; I was at 95 KIAS; at which point I pulled the mixture and did a power-off glide to the runway; floated a long way but landed successfully; and then rolled onto a taxiway before stopping.To be honest; I only feel like I was able to deal with this situation because of my training on the fundamentals of flight. We never practiced anything like it; but I knew I had to slow that plane down by any means necessary; and that picking up speed on approach could be fatal. I am honestly still a bit worried as to what I would have need to have done if the throttle had been stuck full instead of 75%. I suppose I would have tried putting the gear down during a much more aggressive climb. But a faster landing would have been even more harrowing. My adrenaline was through the roof after landing - it took several hours to come down off of it.