A well trained channeler could destroy entire cities if they wanted to. For example, we are told that for channelers of Egwene, Elayne and Aviendha's power level, they could easily tear apart an entire Palace with one blow. With a sa'angreal, a channeler could level a city wall with one blow, as well. When Lanfear decided to attack, in a few moments, she had used relatively simple weaves to kill thousands of Aiel. When we saw Rand fighting at the manor, he was channeling enough Power to decimate tens of thousands of Trollocs fairly quickly, and we've seen 400k Shaido broken by 200 Ashaman (who were using group tactics and not necessarily trying to inflict massive damage). Osan'gar blew up a large section of Rand's apartments at the Cairhien palace, and he wasn't even breathing hard. The implication that I took from this, is that a channeler sufficiently motivated would have enough strength to channel at high capacity for hours. If this channeler is strong enough to destroy a city block with one weave, and they had a few hours to do so, they would have more than enough time to destroy that city and kill all the occupants.

This is further complicated by the fact that the greatest and most destructive forms of the Power do not rely on combat weaves - these would be subtle uses that would manipulate dimensional stability, weather patterns, seismic shocks, massive recurring tidal waves and other large-scale phenomena that we know do not even necessarily require a channeler to be that strong to create. It is not the fireballs, lightning bolts and explosions that are so damaging - it is the uses of the Power that alter people, time/space and fundamental forces of the world. Many of these things wouldn't even be immediately evident. And this isn't even to mention wards that an insane mind might have set to trigger for various purposes. We are actually told as much: "In their insanity, the male Aes Sedai used the One Power to destroy mountains and raise new ones, to drain oceans and flood dry land. Cities were wiped out and civilization completely collapsed during this time, only re-establishing itself in primitive form after the last of the male Aes Sedai were dead." We are not talking about minor inconveniences here - the Damona Mountains and the Mountains of Dhoom were formed because of this seismic activity. Furthermore, the World Sea seems analogous to the Atlantic or Pacific Oceans - this completely vanished. The entire face of the planet was going through upheavals that even a stronger channeler could die in. It didn't matter if you had access to a Circle, sa'angreal and angreal if the area you are in is suddenly flash flooded by waves unleashed by the sinking of a continent on the other side of the planet.

The female Aes Sedai were at a disadvantage because they had to divide their efforts between fixing the things insane male Aes Sedai had caused, trying to survive, protecting populations and then fighting the male Aes Sedai that might have escaped or gone into hiding. By the time the War was over and the Breaking began, their civilization had already regressed into a quasi-medieval state - that was how bad it was. We actually saw how it looked when we saw visions of Solinda, Deindre and the un-named female Aes Sedai herself driven half mad and to the point of barbarity. Quite simply, the female Aes Sedai had no time to "dust themselves off" in order to coordinate a focused effort. The Hall of Servants had been destroyed and large segments of the population were cut off from each other. When we see the female Aes Sedai in that time period, it actually turned out that 'Aes Sedai' as an organization no longer existed, and all we had were individuals or small groups of female Aes Sedai wandering or living with small communities. We know from Tower history that the female Aes Sedai never fully unified. Many died and were lost, and all we know is that some remained with the communities who took to the oceans (becoming Windfinders), others took up the guardianship of communities (becoming Wise Ones and Ayyad) & it was an extremely miniscule amount that found each other, and eventually unified under Elisane Tishar in the westlands. To put this in perspective, the Tower only has influence over a small part of one continent, the Sharans are on the far side of the same continent and the Windfinders for the most part travel the oceans surrounding this continent. Even in the current day, the state of most of the planet is completely unknown. And even now, the White Tower can't be said to coordinate efforts to even mobilize to explore the rest of the planet. Basically, the Breaking is still going on, and RJ's point was that humanity was at the breaking point, and even if the Two Rivers seemed safe and mundane, the wisest of the Aes Sedai knew and admitted that humanity was fading and the White Tower was failing, but no one knew how to put things right.

Basically, the Breaking lasted so long because insane male channelers had set off large-scale forces that the survivors were no longer in a position to easily fix. And this is keeping in mind that, despite the benefits of the Age of Legends, there are groups of the current age channelers that outstrip them in capability. A single Sea Folk Windfinder would be able to match the efforts of a Circle of Age of Legends Aes Sedai with a ter'angreal in matters of weather manipulation and there are weaves that were developed out of necessity due to lost technology. Without access to a dependable Circle, a weather phenomena that a strong Aes Sedai set off would have been impossible for the average Aes Sedai of that time period to fix. And then complicate this by removing every single government, major or minor population center and the stress of roving Dreadlords at the same time (they were still around). It was, basically, a very bad time to live in and it was not easy to find food let alone survive, even if you were one of the fortunate few survivors that had access to Age of Legends training (this is another point that I think you overlooked - by the time The Breaking started, there was at least one generation of young male and female Aes Sedai that never received the full training Lews Therin, Latra Posae, Mierin or the older Aes Sedai had once enjoyed; we are told that training had fallen so significantly that they couldn't even create angreal consistently, even while some specialists still had access to the basic knowledge).