Well, from my experience, it really depends on how out-of-shape you really are as opposed to how out-of-shape you think you are.
I think the best way to really test this other than exposing yourself to the fire early is to ride with a fast group and see how you measure with them.
As far as training is concerned, having a good plan of fierce riding days/mile gathering days/resting days should restore your power in a fair amount of time. Not sure if you can get to your original power within two weeks, but if you train hard, you should be able to perform fine.
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Ride more.
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