Cycling training with gym equipment

[feynman]

I don't race or have any specific goals beyond eventual long distance riding, but I do like improving my cycling performance and overall fitness (with a bike as my weapon of choice).

Our beautiful riding weather just started for the next six months or more, so I am thinking of selling my indoor trainer as I ride outside more, but also joining a cheap gym across the street to supplement, and one more clause here to make it an extra runny run-on sentence.

I might not do this, because I actually enjoy the indoor trainer and its convenience, but I figured I'd ask my more experienced friends their thoughts.

If I do, I'm wondering how best to get better at cycling using generic gym bikes, spin classes (?), etc. I have a fairly good idea of how to approach off-the-bike strength/conditioning stuff, but feel free to give me advice here too. I'm all ears.

I'm currently using my trainer with a repeatable basic Masters fitness plan that scales with FTP, from working with a cycling coach a few years ago. It's a few intervally workouts each week and a longer ride on the weekends. The nature of the interval workouts changes in blocks every 3-4 weeks. I think it's fairly standard as far as general fitness cycling training plans go.

I've already typed too much, so I'll shut up. Thanks for any thoughts on the matter.