I have my Age groupers and Pro's complete this bike set two or three times during prep for an Ironman.
It is important to do it rested and to give yourself time to recover after, as it usually takes a lot out of you.
The focus of the workout is on improving Ironman-specific power and cadence, and the first time you do this session is usually the hardest. But the second and third time you will see great improvement and I can guarantee you will enter your next Ironman with confidence in your bike fitness
30–45 min warm-up spin, 90–100 RPM
1x45K (28 miles) time trial at Ironman watts, 85-plus RPM
15 min recovery spin
5 min (max) food/drink break
15 min spin
1x45K (28 miles) TT at Ironman watts, 95-plus RPM
30–45 min warm-down spin
Total: 90K (56 miles) at Ironman pace plus two hours warm-up, cool-down and recovery
If you've done it let me know how you got on @ Twitter or Facebook