I agree with the others when it comes to walking and good walking shoes. Get in walking shape now, and make sure you leave the brand new shoes home.
When it comes to touring plans, I always suggest that folks make them, but mostly so they can get used to the layout of the parks. The parks are deceptive so knowing the general area attractions and restaurants are situated in will make a huge difference when the visit occurs. Since this is your first visit, spending time on touring plan sites can be a huge help.
As to how to tour, we tend to stay in one area at a time. I make FP to coincide to the park we are in and where I think we should be at that time. I have never been a fan of criss crossing parks even when for legacy FP.
I think that because you have never been, and because you have a lot crammed in to your trip, it may be a good idea to temper your expectations. There is a lot to do in every park, and there is no way to get it all in, even during a relatively slow time of year. Make a list of must do things in each park, and work around them. One day in each park may mean that you need to make some hard choices, especially in terms of evening entertainment vs Rope Drop. In your case, FP is going to be important.
I have never been to Universal so I cannot help there. You did not ask, but to be honest, on a first trip I would not go to both parks. I would save Universal for when I could spend at least two days there, and add another Disney parks day on to the scheduled trip. The MK is at least a two day park IMO, as is Epcot. I dont know what DAK will look like in January, but if there is evening entertainment there, you will want to spend a full day there as well. If DHS adds the Star Wars fireworks display again, well between Fantasmic and the fireworks, you need a full day there too.