The short answer to your question is that your FTP alone will not provide you with any insight as it is a single data point.
If you have a lot of all-out efforts in the 20-90 minute range, you can estimate your 60min from your Power Duration Curve. If not, you could swag it in the following manner:
a. If your FTP is recent and if you have had a steep ramp to it, your TTE (see below) is probably in the 30-45min range and thus likely that your hour power is 80-90% of your FTP.
b. Conversely, if you FTP has been steady and you have been doing a lot of progressive intervals in each of the training zones (SS, Threshold, O/Us), you’re probably in the 90-105% range for the 40k TT.
There are also a couple of great suggestions in the posts above that I’d like ti highlight:
Execute Dr. Andrew Coggan’s 20min FTP test as defined in “Training and Racing with a Power Meter”. Key elements include the early ramp to sustained power and the ramp during the last few minutes. It is how world class TTers execute a TT and Andy’s FTP test is great practice for the rest of us.
Use WK04 to measure and monitor your Time to Exhaustion (TTE). TTE is a great measure of how long you can sustain your FTP. If you do progressive intervals mentioned above, you will see how you can drive your TTE up.
FWIW: When I started SSB MVI my TTE was in the 30+min range; by the end I was 1h 03mins. After doing Andy’s 20min test that led me to raise my FTP, I’m back in the high 30s (the reward for hard work ).