In astronomical or meteorological winter my longest ride has been the Kennett Valley Run (x5) riding from home, which makes about 240km, but my longest ride in 'winter' conditions was a Dinner Dart from Basingstoke to York via Horncastle and the Humber Bridge, which totalled 436km in 24 hours and included 3 spills on black ice and a 5km section walked because the road was a sheet of ice and I didn't want to come off again. A couple of years later, I had another go at a Dinner Dart, got 190km to Market Harborough by which time the temperature dropped to -3C and I didn't want a repeat. I managed a further 200km to York the following day to get my Dinner Dart and then rode back 2 days later from York to Stafford for an After Dinner Dart, where the temperature started at -8C, stayed below freezing all day, and, by the time I could no longer see my cycle computer, somewhere near Uttoxeter, the temperature had gone back to -7C. Struggled with dehydration because of frozen water bottles. After being out of action for a month through bursitis in the hip thanks to black ice, I've given up such antics.
Although the coldest riding conditions I've had were in meterological spring this March, riding from The Hague to Zandvoort and back, in bone dry marrow chilling conditions of -8C and a tasty 30kph headwind all the way to Zandvoort.
