Well, since you said that the horsemen could cross practically half your empire in a turn I assume that you are fighting Tasunke instead of Rhoanna.
first thing I'd suggest, is to simply never build roads near raider civs. Any cities near those civs should possibly only have one main road leading into it, and maybe only a few more to link up resources. (I do have a tendency to spam roads around my other cities, just to get rid of the chance of that goblin waste event).
Protecting your cities: I know the Ljo's strength lies in archery, but I'd suggest you'd consider defending your cities with simple copper warriors, assuming you had copper. They are way more cost effective than archers especially when horsemen gets 40% bonus against archers.
If you know that you couldn't hold your border cities, you might want to pillage those roads you had leading to it while withdrawing. You're elven and running FoL, and those ancient forests would seriously slow those stacks of horsemen down without roads, and they don't really slow you down! (well not as much)
Then you could possibly counter-attack them while they are stranded in the forests, elven units have forest attack bonus after all, or you could use fawns too for extra forest dmg (though they are expensive). If you had copper I'd suggest just spamming warriors (str 4) to toss back at the horsemen, after all they cannot gain defensive bonuses.
Of course this assumes that you have a sufficient counterattack force (usually you'd need close to 1.5x at least to be safe, or have some highly promoted units to fortify the stack if you do not kill them off in 1 turn), else... there's always the PANIC button--March of the Trees!~
You'd better make sure you wipe out the main hippus force with that if you used it lol~
Capturing Cities: Yep I agree that the elves can be quite horrid at capturing cities early on, given their lack of siege. But then again, early on before cats, massed axemen works well too. Elven production in those mega-cities should easily outstrip most other civs, should you decide to go for a brute force method to capture cities. 3 highly promoted archers garrisoned? Just send in 10 Axemen!
Towards midgame, other alternatives open up: you could use fireballs, you could send in 10 tigers to soften defenders up (but make sure you take the city in 1 turn), or if you played as Amelanchier, his calvary is pretty impressive too!
Raider+Defensive is a pretty strong combo for a calvary heavy army. Defensive more for the 10% withdrawal. It allows you to match the Hippus in withdrawal rates, so you could essentially spam horsemen back at them, taking flanking promos to soften their garrisoned units up, and bring some combat trained ones for the kill. Not to mention you could pillage every scrap of land just like the Hippus are doing to yours
Nyxkin are also pretty awesome units, since they share the elven trait, and can rampage through forests swiftly and pull back to relative safety. Allowing you to not rely on roads near border cities but still retaining good mobility.
Lategame you'd have Yvain, druids, beastmasters. Early on the Ljos aren't too good at cracking cities, but they should be able to decimate stacks in their forests with ease (assuming you aren't outnumbered badly).
edit: whoa you had 3 replies in the time I took to type this out lol~