There can be huge variance in January. We've had days where the lows dropped into the high 20s and never got out of the 50s. This past year we wore our hats and mittens to the parks in the mornings, then peeled them off by 11am or so.
On the other hand, we've had some January days (2 years ago) where highs went up into the 80s. In short, it is very unpredictable. Some people don't like going to Florida when the highs are only in the 50s. On an "average" day, the highs will go up into the low 70s, but as I indicated, there is wide variance. Park attendance is usually pretty light, but don't expect to walk on popular rides. Also remember that parks close early, except for Epcot's World Showcase.