In the British army of 1914, a division at full strength would consist of 12,000 men, 6,000 horses and 1,028 waggons. If there were no suitable railway communications, they would march; each soldier carrying upwards of 60 pounds weight of equipment. The standard marching pace would be three miles per hour, covering around 20 miles a day: more in emergencies. Marching four abreast along a single road, a division would extend 15 miles in length, and take five hours to pass a given spot.
It can be seen that in some respects little had changed since the days of the Duke of Wellington a hundred years earlier. The immense number of horses created huge problems of supply for all sides. General Kluck's First German Army in 1914 had a total of 84,000 horses, which needed to consume a thousand tons of fodder per day! Also, one of the factors leading to the failure of the German Schlieffen Plan in the summer of 1914 was the sheer exhaustion of the troops. At one stage Kluck's men, who had the furthest to advance, marched 70 miles in 40 hours. It is hardly surprising that they were forced onto the defensive at the Battle of the Marne in early September.
(Source: Denis Winter: "Death's Men")