Going alone is a lovely way to visit the parks, by taking it easy and taking your time looking around you'll fill up those 4-5 days quite quickly I'm sure.
But the others are correct, you can cover a lot more ground in January, when the park is basically at its most quiet.
However that simply means you can do your favourite attractions multiple times quite easily.

Perhaps consider doing a tour, they're usually quite fun and you can learn a fair bit about each park.
And since they take 90-120 minutes, they'll fill your day right up.

To be fair, an Itinerary wouldn't really be needed, but if you wish to do so, I would start by looking up the busier rides you really feel you must do and plan a route around them.

And perhaps consider entering your chosen park for the day early and snagging a fastpass for rides you want to do (especially on Ratatouille this will be very useful, as it tends to be busy. But it might not be that bad in January).
And I'll agree with the others, be sure to wear comfortable shoes.
