If it was a fuel supply problem severe enough to affect idle speed, you can bet it would really affect running speed when the engine demands even more fuel. Could also be a vacuum leak, in fact check your brake booster to engine vacuum line. You stated the pedal was very hard to press. The brake booster should hold some reserve of vacuum for a couple of power assisted stops with the engine off. When checking the brake booster vacuum hose, disconnect both ends and inspect it thoroughly. Stick your thumb over one end (engine side) and try to suck on the other end and create a vacuum and make sure it's holding vacuum as well. On my 2.6L, the engine side of the hose looked perfect from the outside, but once I disconnected it and looked inside, it was obviously bad. The whole inner hole was cracked everywhere and hard as a rock.