It does seem unlikely at first glance but a closer look reveals that it is not quite as impossible.

If we figure an average distance runner (i.e. not a world-class runner) can cover a mile in about ten minutes (6 mph or 10 kph) then it would take roughly 1,000 minutes to cover the 100 miles, or just over 16.5 hours. If we slow that pace to half speed, about 20 minutes per mile (3 mph or 5 kph) then it would obviously take 2,000 minutes to cover the 100 miles which translates to about 33 hours of travel time. A 3mph or 5kph pace is more or less a fast walk, and not really a sweat breaker. I grant that would be depending on who you are, but since orcs seem to be pretty fair runners (as exemplified from the chapter The Uruk-Hai) and able to maintain a running pace for great lengths, then it seems pretty reasonable that they could have covered the 100 miles between Isengard to Helm's Deep without being too tired to fight at the end of the march. However, moving troops is another matter entirely, and depending on the size of the group, it might have taken the entire 36 hours (even if some of the army was on forced march and others running full out) to bring the entire army to Helm's Deep. But still it is a conceivable feat.

-RR