Hi Jen - in 2012, we visited the Colosseum one day and the Palatime Hill/Forum on a separate day, as suggested above. All 3 sights are huge, and if you poke around every nook and cranny, it will take lots of walking (both on the level, and up and down) and will require a good deal of time and energy.
For the Palatine Hill, we used the Via San Gregorio entrance, as Jeff and Kathy mentioned, and climbed up from there. After exploring (we didn't go inside the museum at the top), we worked our way downhill. There's a network of paths and walkways (some with steps), so you may wind you way downhill, or take the "straightest" route. Some of the paths have excellent views overlooking the forum. At the bottom, near the Arch of Titus, we hung a left into the Forum and the Via Sacra. At the far end, near the Arch of Septimius Severus and the Curia building, there's an exit-only gate that leads to stairs up to the Piazza del Campidoglio and the Capitoline Museum. To exit near the Arch of Titus, you have to backtrack up the Via Sacra towards that arch.