How long to recover fitness?

Question. I have effectively done zero training since early July (2017). I used to have a pretty decent level of fitness with a 4.8-4.9 W/kg FTP. But in the time since then I've put on a whopping 6kg to 72kg. How long would it take to recover that level of fitness and what's the quickest route to doing so?
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I think the sooner you remove the idea of "quickest route" the better you will be. At 72KG you are not exactly a heavyweight so general riding should still be fun, I'd bet with 3-4 months consistent riding, building the volume and intensity at an appropriate rate you'll not be far off. Then you can really start punishing yourself.
Do you have a power meter? Whilst it's not the definitive calculation you could plan the amount of TSS you'll do each week from now until say July the first and see how your CTL looks. Of course all TSS is not accumulated equal but as a gauge it might help.
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Nice response.
There are faster ways but they come with trade offs. So it will depend on the OP's need (e.g. sometimes a racer needs to return fast to make a team deadline or for a particular event and the balance of the season isn't important), why they were off the bike (injury, illness, just not riding) and how much they are willing to trade off, e.g. depth/longevity/narrowness of fitness and/or accept an increased risk of illness/injury for a rapid return?
As to what specifically those strategies are and what's optimal, well that is individually specific and impossible to provide a general answer.
In short anyone seeking rapid fitness progression needs to know what level of increase in their training load over the period in question they can sustain (it's individually variable but we do know the typical upper limits where most are at high risk of cracking), what sort of specific work to include that best suits their particular needs and what sort of work they best respond to. It also depends on what the rest of their life entails, what training opportunity they have, their diet and importantly what sort of recovery they will be afforded.
I'd always suggest following good process, and the fitness outcome will be what it is.
And why set a limit at 4.9W/kg?