I don't think that fitness could be described as a skill set but the knowledge and techniques for obtaining and maintaining fitness are.
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I am a bit of a latecomer to cycling and over the past two years I have gone from being a once a fortnight mountainbiker to riding Gran Fondos to competing in cyclocross, to competing in crits (I'm over 40). This year I was aiming to improve 5% and maybe start getting placings in cyclocross this winter.
Unfortunately, I slipped on the ice at the beginning of Feb. and tore my quad. tendon. A whole month in plaster and I was only allowed back on the bike at the end of April.
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I have an entry to the Maratona dles Dolomites and I had persueded a friend - who had the same level of activity as I had 2 years ago - to enter too. Whilst he has been training since February, in the past month of being back on the bike, I have gone from not managing more than 10km, to riding him into the ground over 2hrs.
Some of this must be down to residual strength and fitness - though my injured leg is still far weaker than it was and nearly 5 months off destroyed my aerobic fitness - I would put a lot of the gains down to knowing how to train to regain fitness, a skills set.