The runners are in the throne room of your battle tower, once you have taken it over (along one of the walls, near where you teleport to using the dragon stone).
Check your alchemist (after you equip any jewellery you may have with a Master Herbalist bonus), and see what ingredients you need for the strongest healing potion you can make, then send the herb runner for it/them. You can ignore the other two, or sent them out for any minerals/gems you may need, or diamonds and malachite ore to sell.
Runners and Tower ?Level 1 of the Healing spell would help you conserve potions. If you have skill points you don't know what to do with you could invest in potion efficiency, but I never bothered with it (except in my first game before getting the battle tower). Some people like the ghost summon, which can heal (though not necessarily exactly when you want it to) or use a fireball attack.
