The answer is (25 + (50/sqrt(2))) miles.

Here is the solution:
Assume by the time the messenger met him, the guy upfront had marched x miles. When the met the messenger might have marched (25+x) miles and the guy upfront might have marched x miles.

After they met, till the end of the day, the guy upfront might have marched (25-x) miles and the messenger might have marched back x miles to take his position (by the end of the day, the last guy in the column might have marched 25 miles).

Putting them in equation yields: (25 + x)/x = x/(25 - x)

From it we get: x = 25/sqrt(2).

The messenger might have marched a complete distance of 25 + 2*x miles in the whole day yielding: (25 + (50/sqrt(2))) miles.