You can always try to leave worldgen running while just leaving to do something else... But at the same time, I find that I prefer to just "shotgun approach" and come up with a half-dozen worlds until I find something I like.
So far as settings are concerned, I honestly tend to come up with settings that produce more interesting embarks than worry about legends in particular, but what I tend to do is to crank up the variance and drop the game's rejection of worlds without large tracts of particular terrain types. You can combine this with changing some of the parameters of the entity's raw's SUPPORT_BIOME (or even the settlement) so that they can spread further. Dwarves in particular tend to get closed in on a single mountain range without being able to spread, so breaking up the terrain, and giving them the ability to scamper across longer stretches of valley to settle more lands, and let them interact with a wider range of different cultures.
I also tend to stop at around 500 years, although that's more just because I don't need it any longer since I intend to play, rather than wallow in Legends, alone. (Not that I don't take an interest in Legends, but I intend to do both.)
Also, oceans are boring and don't support much life, so have less of them unless you're going to make a large world. Even using a medium world, if you take out a lot of the oceans, you can cram more civs in there, and even if a few get wiped out, you still have more. (Plus, fallen empires are interesting, anyway.)
Likewise, there's little point in having more than one cavern, so you can probably clip them down to one or two in order to speed worldgen up a bit. (You can also mod the game to make things that only spawn in the second or third cavern appear in the first cavern layer so you don't miss out on anything if you want.)
[WORLD_GEN]
[TITLE:CUSTOM MEDIUM]
[DIM:65:65]
[EMBARK_POINTS:1274]
[END_YEAR:500]
[BEAST_END_YEAR:100:66]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:0]
[ELEVATION:1:400:2000:2000]
[RAINFALL:0:100:900:900]
[TEMPERATURE:25:75:200:200]
[DRAINAGE:0:100:1600:1600]
[VOLCANISM:0:100:2000:2000]
[SAVAGERY:0:100:1500:1500]
[ELEVATION_FREQUENCY:2:2:4:3:2:1]
[RAIN_FREQUENCY:2:3:1:2:2:4]
[DRAINAGE_FREQUENCY:2:4:1:2:1:6]
[TEMPERATURE_FREQUENCY:2:3:2:5:3:1]
[SAVAGERY_FREQUENCY:2:1:1:3:1:4]
[VOLCANISM_FREQUENCY:2:13:2:1:1:13]
[POLE:NORTH]
[MINERAL_SCARCITY:1000]
[MEGABEAST_CAP:12]
[SEMIMEGABEAST_CAP:28]
[TITAN_NUMBER:9]
[TITAN_ATTACK_TRIGGER:55:0:1000000]
[DEMON_NUMBER:25]
[NIGHT_TROLL_NUMBER:5]
[BOGEYMAN_NUMBER:5]
[VAMPIRE_NUMBER:8]
[WEREBEAST_NUMBER:8]
[SECRET_NUMBER:25]
[REGIONAL_INTERACTION_NUMBER:10]
[DISTURBANCE_INTERACTION_NUMBER:10]
[EVIL_CLOUD_NUMBER:5]
[EVIL_RAIN_NUMBER:5]
[GENERATE_DIVINE_MATERIALS:1]
[GOOD_SQ_COUNTS:8:32:96]
[EVIL_SQ_COUNTS:8:32:96]
[PEAK_NUMBER_MIN:4]
[PARTIAL_OCEAN_EDGE_MIN:2]
[COMPLETE_OCEAN_EDGE_MIN:1]
[VOLCANO_MIN:5]
[REGION_COUNTS:SWAMP:40:0:0]
[REGION_COUNTS:DESERT:10:0:0]
[REGION_COUNTS:FOREST:200:0:0]
[REGION_COUNTS:MOUNTAINS:200:0:0]
[REGION_COUNTS:OCEAN:200:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:200:0:0]
[REGION_COUNTS:HILLS:200:0:0]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:100:0]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:4500]
[CAVERN_LAYER_COUNT:1]
[CAVERN_LAYER_OPENNESS_MIN:10]
[CAVERN_LAYER_OPENNESS_MAX:45]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:10]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:30]
[CAVERN_LAYER_WATER_MIN:0]
[CAVERN_LAYER_WATER_MAX:50]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:12]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:6]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:8]
[CAVE_MAX_SIZE:80]
[MOUNTAIN_CAVE_MIN:25]
[NON_MOUNTAIN_CAVE_MIN:50]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:16]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:264]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:100:600:100]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
This example is skewed to have less oceans (more like seas, really) and large mountain ranges, with more grasslands and forests with occasional spots of mountains.
You can see from the example, you can wind up with a fairly well-inhabited world, with only a flukish band of low habitation a little below the middle simply due to the human entity spawning in the southern half of the map getting suppressed by the elves before they could really take up the grasslands. You could easily put more entities in this world and have them fight it out for space and make some extra interesting fallen or conquered entities.
(Also, terrain in general is more broken up because I prefer to embark on "corners" where biomes meet, so more varied and fractured biomes result in more "corners" where those smaller biomes meet.)
(Also, also, warning - that's from 0.42. I don't THINK there are any changes to the worldgen, but you might want to compare to make sure...)