Disclaimer: I'm not a seasoned racer. *But* I train hard, race occasionally, and do the weekly hammerfest with the club, which is faster and harder than a lot of races. Well, cat 5 races, at least.
This is my second year of rigorously doing base. Effort target is exactly what RiphRaph said. I target 135, which means I'll spend most of my time between 135 and 140. That's actually about 80-85% of my AT, but it's a completely easy aerobic level *for me*. Some online coaches recommend mixing in occasional harder efforts. I don't until what I refer to as base 2. And I still do recovery between base rides, which are even easier. Since I can't log major miles (time and trainer tolerance constraints), I aim for consistancy... at least an hour, lately more like 1.5 hours, on the bike 6 days a week, at least one long ride on week-ends.
Season-wise, I started mid-October this year... early. I was going to try CX, but my performance had been dropping off for a month or more and I was just cooked, mentally and physically. First race around here is the end of March, so that's when I target being race-ready, and work back from that. I plan to continue strict base through December. In January I'll start ramping up intensity (base 2), though still primarily aerobic/tempo (developing slow-twitch/fat burning), and allow myself a little latitude for charging up hills, etc. (and, boy, will it be welcome). February I'll start mixing in some anaerobic efforts, and in March will start ramping up with some serious intensity, to be ready by the end of March.