Fuel Pedal is tough to push

Buy some heavier boots!

Seriously though, depending on the auto transmission design, the throttle pedal may have an additional cable which controls when the tranny changes gear, depending on how hard you're trying. If so, make sure that this cable moves freely. If it is sticky, it could affect the feel of the throttle pedal.

For what its worth, I had a rather hairy experience in a 760 GLE TD. Had bought it the month before. Was coming over an overpass of a local freeway with traffic front and back, and both sides. The hill had a sharp incline... too match traffic I pushed the accelerator farther than usual... and it stuck down, with slack, meaning I couldn't pull it back with the foot. So with a combination of brakes and ignition key I kept the vehicle roughly in pace with the traffic... most people would have had an accident at this point I think, as the traffic was around 40mph, with tight spacing between cars.

Anyway... got it to safe point and pulled over. Tracing the engine side, I found the throttle plate that the accelerator cable was controlling. It was spring loaded, which provided the movement back through the firewall. It had caught on a recently done replacement of all hoses (before I bought it). One had to depress the accelerator beyond a certain point for the plate to catch on water hose which was routed over the plate. A zip tie fixed it.

Point is, do a bit of checking as to what might be binding. Could be the cable itself, could be what the cable operates. You may be able to tell with some quick checks... in your driveway. If its the cable binding, either due to lack of lube, or whatever, you may have to replace it if you can't force lube into it. If there is a kink in the cable (probably not), you'd also have to replace it. But there is a good chance you can figure out the problem. Could also be the spring return is caught or too stiff.

old cables can cut a groove in the plastic liner. Check and if necessary replace

I've had two scary experiences related to this with two different causes:

1983 240 Turbo: The pedal is connected to the throttle body by a cable that runs through an threaded plastic assembly (a short plastic tube ~2.5cm long, threaded so you can turn it to adjust) mounted on the throttle body bracket. From age and use this plastic tube broke and the cable stopped passing straight through and the resulting kink in the cable was too much for the spring in the throttle body to overcome. Result: pedal hard to push and slow return to idle speed.

1989 760GLE: While driving I had cause to step on the gas pedal and push it all the way to the floor. This was outside the normal range of use and caused the throttle body to turn out of the well-worn range and get stuck in a "wide open" position. Once I got the speeding-out-of-control vehicle to the side of the road and got under the hood, I was able to put my hand on the throttle body and give it a bit of a turn at which point it snapped closed. A soaking in WD-40 freed it up at the top end of its range. Very scary.

Based on this I'd say start at the throttle body and make sure it's free and smooth in its operation and then work your way back.

Good luck, blurp

And sometimes they pop off the pully and that makes them stiff.

Also the automatic pedal may be a bit harder to push since it also drives the kickdown linkage for the transmission, it shouldn't be a lot harder though.