What? It's 230 miles from Cannae to Rome. Assuming they marched 10 hours a day at 4 mph, thats 40 miles per day. It would take them around 6 days to reach Rome. Even if they marched for only 8 hours a day, it would take 13-14 days to reach Rome.
You are vastly overestimating the speed that armies can travel in a sustained march. The best sustained advances in history – the Mongols, the blitzkrieg, Sherman’s march – averaged 10 miles per day. A forced march might get this up to 30 miles per day, though even 20 miles per day will result in exhaustion and straggling. That maximum total distance that an army could force march is probably 60 miles in total over the course of two or three days.
Getting the whole Carthaginian army to Rome would have taken three weeks at best, but Maharbal did not suggest the entire army moving on Rome, just the cavalry. Using multiple mounts per rider, Maharbal might have gotten to Rome with some of the cavalry in about 5 days, though it probably would have been a few days longer. It wouldn’t be a large enough force to siege Rome, nor would they be ready for battle, but Maharbal wasn’t suggesting that, either.
Rome had their back against the wall in actual history. Major portions of their defense were slaves and criminals freed if they promised to fight and there weren’t even enough weapons and armor for them to be properly equipped. Intimidating the Romans into surrender before they could return to a Fabian strategy was Carthage’s best chance of success. That’s a bluff, but on a certain level the entire war was a bluff by Carthage – they lacked the troops and especially the naval forces to have much chance in a sustained war.
The best time to bluff is after winning a big hand and I think that’s what Maharbal was suggesting. Rome was already near panic. Now imagine if the first news of Cannae was from Carthaginian cavalry. Who appear to have reached Rome with impunity. Especially if the Carthaginians have some Roman standards or severed heads of leading Romans to throw onto the dirt outside the city. Throw in an offer to spare all lives if Rome surrenders and their weight in silver to any man who opens the gates, and there as a good chance Rome would fold.