Becasuse a 45 min spin class makes you feel like you’re Schleck after chaseing Contador up a moauntain after he’s been eating the special sausage.
There’s no way I could push myself THAT hard on a ride, things would get in the way like downhills, or traffic, and all out efforts to almost blacking out aren’t generaly a good idea on your own.
Its kinda like going out with the local chain gang, only with more ‘view’.
Not a substitute for riding, but if you just want to get leg strength and go fast, quickly then 45min Spinning easily eclipses 1hr30 Singlespeeding.
Once my knee’s recovered I’ll probably go back, used to do it 3x a week (monday, tuesday, thursday) where tuesday was back to back sessions with a 15min ‘rest’ where you can regain some composure and fill a water bottle.