I did a winter of these 2 x 20s at sweetspot (90% of max hr). That was the most/ only highish intensity training I did. The rest was just base miles/ club tuns. Got to say it did little for me. Not intense enough. What rob is suggesting is a bit hardcore but in my opinion better to do that fitness gains wise, even if you cannot hold on for the full 20 mins, than to do 20 mins at sweetspot. Depends what other training you are doing in addition of course. Davey for example will be getting some very high intensity short intervals on the track, which longer sweetspot intervals probably complement nicely.