Possible cons of excessively long microphone cable (XLR)
An XLR cable is a "balanced" cable type, which means that it is inherently noise-canceling. For the most-part anyways (nothing is perfect.)

It achieves this by not just sending one signal down the cable, but sending two identical signals - except that one is electrically reversed or opposite from the other. This is called a "differential" mode - when one wire goes positive, the other goes negative. This is beneficial because any extraneous noise that is picked up by the cable is picked up evenly by both wires. This "common-mode" noise is easily rejected. 

So long story short, no, a hundred feet of XLR cable probably will make little difference in noise. The signal strength might be reduced slightly due to singal losses, but it should not be noisier because it is long.

That said, XLR cables are great at eliminating the E part of EMI - Electro-Magnetic Interference - or things like radio waves, the "usual" sources of interference. But they are not as good at rejecting the M part - magnetic interference. Luckily, few things produce a magnetic field large enough to be concerned about, so unless you have this cord wrapped around a vacuum cleaner, it shouldn't be a problem.