The standard amateur sportive build is 12-16 weeks split across four phases. 4 weeks of base (long zone 2 rides, low intensity, building aerobic capacity), 4-6 weeks of build (twice-weekly sweet spot or threshold intervals layered over the long ride), 2-3 weeks of specific work (race-pace simulations, climbing repeats, tested fuelling) and a 1-2 week taper. This isn't novel — it's Joe Friel's framework from The Cyclist's Training Bible and the structure underneath every Roadman event plan.
Mountain sportives need a longer runway. The Étape du Tour, Marmotte, Mallorca 312, Maratona dles Dolomites — these events have climbing demands that don't compress into 12 weeks for most amateurs. A 16-20 week build gives you the durability work (4-5 hour zone 2 rides, climbing-specific sweet spot at the back end of long rides) that decides whether you finish strong or hang on. The harder the event, the further out you start.
What to do inside the under-8-week window. This is damage control rather than build. Stop adding intensity — the adaptation window has closed and any new threshold work just adds fatigue. Instead, ride consistent moderate volume (3-5 rides per week, mostly zone 2 with one weekly tempo session), peak your fuelling protocol so race day isn't the day you test 90g/hr, and rehearse pacing on a long training ride at goal sportive tempo. You won't gain meaningful fitness in 6 weeks, but you can absolutely lose a sportive in those weeks by overcooking it.
The aerobic base point most amateurs underestimate. A structured 12-week block delivers full gains only if you arrive with a base — meaning 8-12 weeks of consistent zone 2 volume already in your legs. Riders who jump from 4 hours a week into a structured threshold block typically plateau by week 6 because the aerobic engine isn't ready to support the work. If you're starting cold, treat the first 4-8 weeks as base-only and add 4 more weeks to the calendar.