Edited for clarity
When travelling in large masses, armies use formations to simplify the logistics and manage the risks of contact with the enemy. Due to differences in their anatomy, human and naga formations would differ somewhat, but not hugely.
Human formations are typically organized as a grid in ranks (rows) and files (columns). The spacing between soldiers in a rank is called "dress". A normal dress is established when the front rank of formation members can touch the shoulder of their nearest neighbor with the finger tips of one hand. "Full dress" is when the front ranks can touch only the fingertips of their neighbor with either hand.
When travelling in a column on land, naga could be close packed in each rank, like similarly sized humans, but the ranks would be sufficiently far apart from each other to allow for the tail. If for any reason it was desirable to make the formation more compact from front to back, the spacing in each rank could be increased up to full dress, allowing the next rank to come up part way between the tails of the rank ahead.
Due to the geometry of the formation, naga would be able to execute a wheel maneuver, but a turn would be more difficult (imagine getting whipped with the tail of your 200 lb neighbor in a quick turn). Their fancy parade drills would have some maneuvers that would be difficult for human formations, but be geometrically incompatible with some executed by their human counterparts.
On land, they would use many of the same formations that humans do, with appropriate modifications. Porcupine, Turtle, Phalanx, are all reasonable. While backing up is not a preferred action of snakes, it has been filmed in nature, so we can assume the full standard range of mobility of these formations.
A flying V (or wedge) is used to smash enemy formations. It started out as an infantry formation, but is still used by modern aircraft and tanks. For the naga, it maximizes their ability to bring weapons to bear and to defend each other, while avoiding entanglement of the tail with one's neighbor.
Cadence, which is important for keeping members of the formation from tripping over each others' feet/tails, would be different and more subtle for naga than humans, but not less important. Humans marching in cadence can collapse bridges. Unlike humans naga would not have to break cadence when crossing bridges.
Edit for new information from @G0BLiN re black mamba's
Based on a naga having the same weight as a human, their mobility on land may be limited. If 50% of their weight is in the human torso and head, then Naga would not be able to go long distances because their weight distribution acts as a natural brake and limits the amount of traction their tail can provide. However, if only 30% of their weight is in the humanoid torso and head, they could move quite efficiently.
In open water, their formations would be swift and efficient. For long distance open water treks they would use a flying V formation that minimizes hydraulic drag on the formation as a whole and allows the tailing members to rest in the wake of the formation. Ranks would normally be rotated so that everyone has a chance to rest in the wake of the formation. However a commando action might use grunts to lead the formation the entire way, keeping the commandos fresh for lightning strikes and deadly efficiency.