I kind of put off (ok, for a long time) checking my coolant, and wound up needing to add faucet water to my truck when it went from C to pretty damn near H one day. To be honest, I thought this was weird, because normally it's at about 30% of that gauge, and it got almost to the H.

As I said, I added faucet water (quite a bit), to my '97 F-150. My question is, do I now need to flush my radiator and add the normal 50/50 anti-freeze/distilled water, or should I be ok?

you need to flush any way if you have a 4.6 or a 5.4 . they have cooling issues and you should be changing the green coolant every 60K . the motorcraft gold will run longer . These engine have internal erosion issues with green coolant . I have seen more than one pinhole in the heads from the inside out , some can be welded some need new heads

Never use tap water for a radiator, distilled only.  Flush and add 50/50 mix.

if you have a water softener dont sweat the tap water. Its just the extra minerals that can be present in water that can cause problems. If you have a water softener or some sort of filter theres no need to buy distilled water. As far as your mix ratio, if your in doubt just swap it out. Spending a few bucks on coolant is allot cheaper than having a block welded or replaced if it freezes...

There's no need to run anything other than water in the warmer months. Anything else generally always makes it run warmer than it will with water.

Pure water boils at 212 °F at sea level. By adding a 50/50 mix of water and coolant the boiling point is raised to more than 226 °F.  I'd rather not boil over.

Lets not forget that Antifreeze also does double duty against not just corrosion and freezeup but against cathodic reactions between iron and aluminum. Without it, the aluminum becomes an anode and "sacrifices" itself to the iron causing "honeycombing" and damage.

Always use at least a 50/50 mix.  Especially in Ohio where freezups are a real issue. This is why antifreeze "wears out", the chemicals used to slow the galvanic/cathodic reaction become depleted and the aluminum begins to pit and "dissolve"

Boiling point is higher (above your engine overheating) at 15 psi. I wouldn't run pure water without at least water wetter for corrosion protection.