The LEP experiment's limits on the Higgs mass were set by looking for a process where the experiment would have produced a Higgs boson together with a Z boson. The highest energy they achieved for the electron-positron pair which annihilated to make Z,Higgs was 209 GeV, and that was only achieved in the last months of the experiment. Since the Z boson mass is 91 GeV, the highest energy Higgs boson which could be produced this way would have a mass of 209-91=118 GeV. Some of the energy is always lost to getting the Z and Higgs to move apart from each other, so in practice the limit they could achieve was a little lower than this, 114 GeV. By running much longer and accumulating statistics they could have extended their reach a little bit, perhaps to 116 GeV; but not to 124 GeV. That could only have been achieved by significantly increasing the energy of the beams -- which I believe they had already pushed as far as they could.