Runners are basically just slave warriors with -1 HP, -4 damage (spear vs. trident), 1 less encumbrance and 3/16 move vs. 2/11, at the cost of one less resource point.
They're honestly not that bad of an alternative to the slave warriors, as they're marginally cheaper and have less encumbrance. The 3 map move may not seem great since you don't have access to any slave master mapmove 3 commanders, but if you put a pair of boots of long striding on a taskmaster you get exactly that... MM 3 with slave driver. Theoretically you could turn that into a quick-response task force or something, I'unno. Point is, stat-for-stat, runners are honestly pretty decent against lightly-armored enemies, since with their 2 attacks and 3 encumbrance they can tire out larger forces fairly well if they stick around long enough. Little expensive at 13 gold, but they can be levied quickly thanks to only costing 2 resources a pop.
Definitely not the frontline bruisers the elite warriors are, but there's some use for them.
As for expansion strategies, I've always kinda liked the poison slingers... They chew through heavy armor like nobody's business, and the AoE on the pellets helps against shields/general inaccuracy. Bloody expensive though, but still gonna be available a lot earlier than foul vapors.