Possibly the defining characteristic of Julius Caesar's legendary status as a general was the speed with which his army was able to operate- what accounted for this? What was he doing that other Roman generals weren't? Do we know how he was able to pull this off?

Excerpt from /u/Teizke :

He was in fact a believer in forced/loaded marches as seen in his campaigns mainly in gaul. He was able to do this because he was so charismatic and loved by his troops. He would often sever[e] his baggage train in order to march faster which is a very risky move that can result in the cutting off of the armies. It almost always worked out for him but more often than not for other commanders

/u/XenophonTheAthenian

Both posters here correctly mention that Caesar would often march with no baggage and often resorted to forced marches, but they neglect Caesar's use of night marches, which was a key feature of his strategy of movement ......

Caesar's marches were always rather quick, since he marched for longer than most armies

[Plus of a couple of famous marches that they were more like dashes/flanking manoeuvres that left a large portion of the army to catch up and left even the vanguard tired and having to rest after they got there...]

... It's worth noting that not all Caesar's troops got there at the same time--it seems that a significant portion of his army fell out of the march and continued as best it could, to be collected over the course of the next couple days and allowed to rest.