Again just personal experience, but I had to miss 8 weeks of running following a bout of pneumonia (early December 14 to early February 15) and I've now been building back up for 6 weeks since the layoff.
Raced for the first time since early December this morning and ran a 10K in almost exactly 4 minutes slower than in the same race this time last year (48 minutes compared with 44 last time).
Felt about right; fine at "cruising speed" but much, much harder to sustain any kind of pick-up in pace. As a rule of thumb, I think you need to be training at your previous level for at least as long as your layoff (not counting the building back up period) and if you're over 50 maybe add 50% to that as it does take longer to adapt to training full stop.
Leaving aside the rights and wrongs of the current Twitter "spat" between Mo Farah and Andy Vernon ( and I think AV's initial remark was pretty innocuous and Mo shouldn't have reacted as he did) I was struck by reading that Andy Vernon only earned approximately 20 - 25K from his athletics career in 2014.
Considering he is the 2nd best distance runner in the country (possibly in Europe) this is a paltry reward for the raw talent and commitment needed to reach such a high standard.
I imagine it's not much different in some other individual sports (the earnings gap between Andy Murray and the other male tennis pros must be huge), but athletics is now a professional sport and this isn't exactly encouraging for exceptionally talented young athletes who are maybe hoping to make a career out of running. It seems you've got to reach Olympic/World Championship medal standard to earn a reasonable living (in what is likely to be a very short career at the top).