Massively difficult, in the ordinary sense, although all countries faced such difficulties and the US perhaps had no worse a time of it than other states with smaller peacetime economies.

The US actually started converting to a war economy in May 1940, a year and a half before it entered the war officially. The economy did not get anywhere close to fully converted until late 1943 or perhaps early 44.

The physical difficulties of retooling factories, training new workers and retraining old ones to new task took a huge effort. The political difficulties were perhaps much worse, and not particularly well handled by the US political system.

Basically a fully converted war economy is a command economy. The military makes plans for the weapons it needs and then the government tries to get them produced, while also keeping the populace fed, the infrastructure operable, the army staffed and everyone happy enough to keep doing their part in the war effort. A planned economy takes a lot of organization to run, you need to know how much food you need and where you need it, how much raw materials each factory should get, how much of each kind of labor you need and how you're going to house and feed those workers. It's like a big crossword puzzle where every answer effects the decision next to it. In peacetime, the US has relied on the market to solve this problem, but in wartime the Military is in effect the major buyer of goods, and therefore administrative machinery must take over.

The US political system lacked this administrative machinery and had a very difficult time creating it. Congress mostly ceded the job of organizing it to the Roosevelt administration by the War Powers Acts and other laws, but was very suspicious and interfering when it came to important decisions. Farmers, Business Owners and Labor Unions all lobbied Congress and the administration intensely for a wartime economy that favored their interests. The New Deal and Pro-business factions also were very suspicious of each other. Both Roosevelt and Congress were concerned that putting the war economy powers in an existing government agency (Treasury or the War Department, for example) would make those powers difficult to recover in peacetime. So the Roosevelt administration created a series of various agencies to run the war economy, Office of Emergency Management (OEM), Supply Priorities and Allocation Board (SPAB), War Production Board (WPB), Office of Price Administration (OPA), Board of Economic Warfare (BEW), National War Labor Board (NWLB) and perhaps some others that I should mention.

Despite all the political issues surrounding the manpower and resource allocation problem, the United States had a few things going for it that made it's switch to a war economy both easier and more effective than other countries. Firstly, in 1940, a lot of workers were still unemployed because of the depression. These people were more willing to adapt and take the new defense jobs that opened up rapidly than a fully employed workforce might have been. Secondly, the auto and manufacturing talent pool was vast because of the peacetime industrial might. While a warplane factory didn't necessarily share the same machine tools as a car factory, the people who knew how to run an assembly line could quickly create new facilities to produce defense products. Detroit was full of mass production experts in 1940 in the way that Silicon Valley is full of information technology talent today. This talent pool (along with relatively better access to diverse raw materials) enabled the US economy to be more nimble than other powers that were converting or had converted to a war economy.

Source: Maury Klein, A Call to Arms: Mobilizing America for World War Two