To start with my milleage was only about 35 a week (I was 2.5 stone heavier and was mainly on the heavy hybrid) but I've upped it over the last 6 months to 100+ a week. I try to do at least a 40+ every fortnight, but always do at least 2 30's per week.
Recent prep was really keeping going, I did a 55 last Saturday, a 20 on Tuesday and a sedate 30 on Thursday, then nothing until today. Other than that nothing special. My time wasn't exactly lightning though, there were loads well faster than me. I did find the fact that I've ridden all through the winter prepped me for the wind and rain. There were lots moaning about the rainy wind and I felt lucky to be used to it.
My plan for the 100 in July is to do the same but extend the distances out. Making it an 80 once a month, and what are now the 30's will be 50's. Still a bit daunting though.