Most amateur cyclists overestimate the FTP they need to finish a sportive and underestimate the durability they need to enjoy it. A sportive isn't a 60-minute time trial — it's three to seven hours of sustained sub-threshold work, where the rider with the higher 4-hour power wins, not the rider with the higher 1-hour power.
The honest target ranges look like this. For a flat-to-rolling 100km sportive, 2.8-3.0 W/kg is enough to finish comfortably, 3.0-3.3 W/kg gets you a strong age-group finish. For a 160km sportive (Wicklow 200, Ride London 100), add another 0.2-0.3 W/kg to those numbers. For a serious mountain sportive — Étape du Tour, Marmotte, the bigger UK climbs — you want at least 3.5 W/kg before you commit, and 4.0 W/kg if you want to ride strongly through the back third rather than survive it.
But here's the thing the FTP number alone hides. A rider with 3.3 W/kg fresh FTP and good durability — meaning their power three hours in is 90%+ of fresh — will outride a 3.6 W/kg rider whose power collapses after two hours. Dan Lorang and the World Tour coaching world call this 'fatigue-resistance' or durability training, and it's increasingly the focus of serious amateur prep. Long zone 2 rides build it. Sweet spot at the back end of long rides accelerates it.
Two practical things. First, fuelling decides whether your FTP shows up on the day — 60-90g of carbs an hour from minute 30 is the floor for any ride over two hours. Second, pacing decides whether you finish strong or blow up at km 130. The Sportive Preparation guide on the site walks through both. If your FTP is in the right range but your durability isn't tested, it's the durability that needs work, not the threshold.