I was getting 20mpg when commuting on a highway, but going from 6700' up to 8900' and then down to 4500' one way. But I normally topped out at 60 (and likely averaged 50) going up the hill. Later it dropped to 19mpg, likely when I started driving up the hill and around the corners a bit more aggressively with the 5 speed.
Stock type Fram air filters are cheap, so make sure yours is clean. Plugs are cheap, and it's sometimes good to put a fresh cap and rotor on, though I'm not sure if I have in the past 60k. I put the fancy platinum plugs in mine since they were on sale when buying them, but copper should do just fine.
I don't know if my aftermarket high-flow cat and muffler helped. I only replaced them as needed (stock cat was stolen, and muffler rusted out).
For both acceleration and mpg, get rid of any extra weight you're lugging around for no reason (do you really need the 50lbs high lift jack when on a freeway commuting). It could help a bit to use 0W-30 synthetic. If you're more worried about mpg than acceleration, accelerate slower and reduce that top speed a little.
Of course road tires would help too, but where's the fun in that. But you can keep them inflated or even over inflate just a bit in dry road conditions.
Aside from going up a steep hill at altitude, especially when loaded down (actually did ok with a 1100 lbs of rock going up and over 8900'), my 4runner does pretty well. But when switching out from a V6 acura to a v6 4runner many years ago, I knew I wasn't going to win any 0-60 contests.
