Spinning maybe easier on the knees, but you need to make it a habit to stand up once in a while. Nobody wants to talk about cycling bone loss but here it goes again: Cycling while seated is a non-weight bearing exercise and as such will cause accelerated bone loss. You need to be standing up pedaling to make it weight bearing. Or sprint like heck every so often.
https://www.trainingpeaks.com/blog/w...an-improve-it/