Never been to spinning class but they look torture and I am sure they work but getting out for 30 miles is the best way to go, even on fairly flat roads you have wind and slight gradients to vary the work load plus you will work your core better on the bike. I would suggest do both, road miles as often as you can, spin class when dark, cold and wet...oh that's year round this year. 30 miles at 20 mph is really good going, so I would say that has to be much harder than 45 minutes spin, cooler on you as well, indoor stuff cooks me!