Definitely trail runners for me too, or just plain road type running shoes for what you're doing --- light, breathable, and lots of cushioning.
You might consider getting a different kind of shoe insert, and if at some point your feet are just tired or ache --- and you have more miles to do that day --- swap to an alternate insert that might change pressure points on the foot. Pretty light weight and if you find you aren't using one set, can always throw them out along the way. If using running type shoes I would also make sure to start with a new pair. You want the shoes in good condition, and to cushion well.
That all assumes that you're not carrying a really heavy load on your back, however! People who advocate boots are more often I think carrying heavier loads. If you're really staying near highways and you're not too picky about what you eat, you at least don't need to carry too much weight of food, though there can be some long, lonely stretches of highway with no amenities for many miles.
How long to hike each day? Hiking on the highway, it's partly about where you can reasonably stop ... for quiet and perhaps shaded breaks, for meals, for getting water somehow, and most importantly for sleeping ... somehow. After that it's just about how fit you are, how heavy your pack, how many miles you need to do based on any schedule dynamics ...
I tend to think in terms of miles per day rather than hours, at least in late Spring to early Fall when there are a lot of hours of daylight. But this too varies a lot by the individual. On more normal (away from road) hiking trails, a lot of people are out for the experience, not looking to do a whole lot of miles per day. Others --- often including me --- do longer distance trips where for whatever reason it makes sense to crank out more miles. After I'm up to speed (a couple of weeks on trail typically), I'll be looking to do 20 - 25 miles per day on decent trail. On highway can crank that up to 30 mpd. But this isn't typical and partly reflects that I don't LIKE highway walking and usually want to just get through it and back to actual trail.