I have an '01 GLE Altima since new and your highway gas mileage is the same I have experienced. I have similar mileage as your's, about 81k. I have an automatic with lockup torque converter. In local driving, not heavy stop and go, I have almost never been able to top about 23 mpg. It colder weather it's more like 20 to 22. Extra easy drivng seems to make little or no difference. I have hit 27 and 28 mpg a few times on long interstate trips.
I have heard that the 205- 55 -16 tires on my GLE are known for reducing mileage from the standard 15 inch model. ( 195-65-15 ??). A friend has a 2000 Altima with standard 15 inch wheels/tires and a manual trans. He does a bit better than me in local driving and easily hits 30 on interstates. I used to run 35 psi in tires, but found that lower pressure, about 32-33 front and 31 rear gave much better even tire wear and best handling. At least with my tire size. I had a reputable tire shop tell me the same thing. I think that Nissan specs about 31 front and 29 rear.
I replaced the plugs, distributor cap and rotor around 70k. They're supposed to go about 103k or something like that, but I don't really buy that. Not for optimum running, anyhow. None of the parts were completely trashed, but worn enough for replacement. My gas mileage picked up a bit after the ignition parts were replaced. I think about 12-15k for an air filter change is about right.
I used to use premium fuel, 92-93 pump octane around here. Since prices went crazy the last year, I have switched to mid grade 89. I have repeated the results many times in getting about 10 to 15 % better mileage with the premium. I suspect ( but do not know for a fact) that Nissans must have pretty generous ignition advance limits if the knock sensor doesn't reel it in.
I had the same results with a couple of early 90's Sentra SE-R's I used to own. Those Sr20DE engines were great and easily turned in 36 mpg highway with manual.
The 2.4 is really an old rear drive Nissan truck and 240SX engine adapted to be inexpensive front drive Altima power. ( '93-'01) It's a solid and tough engine, but not the last word in powerful, efficient design. Reading of all the problems with the newer 2.5 Altima engines in the 2002, on, I am glad I have the old 2.4
That is, if I don't get the dreaded leaking intake manifold gasket!
I'd go with new plugs, etc. and airfilter if it needs it. Likely you will pick up some mileage.
Good luck, hank2