I had a shoulder op on 18th Dec and arm in a sling until start of Feb and first rode bike on 6th March. I reckon I missed out on about 80 hours riding including commutes, road sessions, Gorricks and Thetfords etc). I did do 4 x 1 hour turbo sessions in Feb but fairly half hearted. I also walked up escalotors and stairs that is something you presumably can't do.
On 6 march I felt ok in terms of getting up short (200 metres etc) hills but felt there was no endurance and had no wish to ride beyond say 2 hours. I did HONC on Arpil 11th and got on fine, not as quick as year before but riding with a slow friend so not strictly comparable. My commuting bike is SS and I was able to turn that over OK.
By end of May I was marginally less fit than year before judging from results in tour of wessex.
Overall I was amazed that all that hard winter riding counts for so little. No that it will stop me winter riding next year.