The picture above illustrates very well what120 minutes threshold spin cycling does to you. Your legs go numb from lactic acid and screams 'STOP PEDALING!' while your trainers/instructors screams 'Whatever you do, DON'T STOP PEDALING!'. You know you can do it, it's just about getting your mind set on convincing the legs to keep going. Learn to love the burn, and all that jazz.
Afterwards your mind seems to slip into denial and you find yourself thinking 'that wasn't THAT bad' while your legs are still shaking. I don't know it it's insanity or dedication.