A speaker's rated wattage is a maximum value. It is literally the most energy you can pump into the voice coil before it melts and breaks. It is completely irrelevant to ANY discussion of sound quality or even amp compatibility UNLESS you intend to use it with an amp that can output a wattage figure higher than that value. Even then, you will be perfectly safe to use it in that situation, providing you don't actually ask the amp to provide power in excess of that value. So for example, if you have a 100watt speaker and a 200 Watt amp, yes, you CAN blow the speaker with it, but unless you turn the amp up to that level, you still wont.
Far more worthwhile a spec is speaker sensitivity. This is what determines how loud your speaker will go when fed a certain wattage. Most guitar speakers are very sensitive and will provide in excess of 95DbA when fed with a single watt of power. For context, 100DbA is the legal working noise limit I the UK, and working in environments that exceed that level requires ear protection be provided.
Resistance is a different topic, but ultimately on a 1x12" just match them up and you'll be fine. More complex rules come into play if you intend to use cabinets with multiple speakers in series or parallel.
As for running the speaker in - this is a very hotly contested topic and I'm inclined to believe neither main camp is correct. It is possible to break a new speaker by running it too loud from new. This is because the suspension of the speaker is stiff and therefore the cone is more difficult to move. A voice coil being fed current but being restricted from moving, will dissipate the extra energy as heat, and melt itself.
That said, the suspension of a speaker is the only part designed to move during operation and should become flexible with very little time. After that, the idea the sound will change is dependent on some very suspect ideas about vibration permeating through the cone structure. Any running in time over about 30 minutes is likely to be more placebo than actual settling down of the sound. Concern yourself more with gradually ramping up the volume until you hit the level you intent to use most frequently and then just play as normal.
Lastly, the idea of "being able to drive" a speaker is a fuzzy one. As mentioned when I spoke on efficiency, most loudspeakers used in guitar amps will run very loud with very little power. The idea that a speaker needs to be properly driven comes from the fact they're a complex system dealing with complex sound.
The efficiency is rated at 1watt and a 1khz signal. Thing is, a speaker can be more or less efficient at different frequencies, even presenting different resistance loads to the amplifier outputs. When a speaker is being "driven properly", what is meant is that the amplifier is capable of meeting the demand for current at any of the frequencies required.
This IS a function of the amplifier's power output, but ultimately, if the speaker needs a max of 10watts output to drive all frequencies and combinations of frequencies equally well at a given volume, using a 20 or 30 watt amp that is otherwise the same won't affect the sound one bit. All it will mean is you'll have more headroom to go further up in absolute volume. You will NOT, repeat NOT be "driving the speaker better". Just enough in this instance is all you'll ever need.
All this of course, forms the basis of why in both instrument and stereo amplifier design, you will almost never, ever need as much wattage as people tend to think. Its a fantastically poor way of judging the performance of an amp. Sadly we seem to be stuck with it as nobody but NAD amplification seem to have any interest in a more useful measurement being made standard.