The one thing I've never been comfortable with is Mo's UK 1500 metres record. Taking 5 seconds off your PB at age 30 and running faster than Coe, Cram or Ovett ever did - 3 MD specialists and acknowledged all-time greats ...?
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.