To give you an example
@HamboCairns - I'd been riding one day, on roughly 3 out of 4 weekends for the last few years. My fitness improved, but plateaued quite quickly and never really improved any further. I played around with a smart trainer during COVID lockdown, then gave up after a month or two because it was boring as anything. I did learn during that brief stint on the trainer that what
@Minlak is suggesting was true though, that my brain was telling me I was done
long before I was actually done. On a trainer you can really force yourself, because it doesn't matter if you're slumped over the bars and feel like you want to spew. You realise after a while that your body was just lying to you because it's not used to it and is wondering WTF your're doing to yourself. Psychologically, you've got to be tough to stick with it. So I stopped. Many months later once our lockdowns eventually eased, I started riding twice a week with
@fjohn860 - roughly around the start of summer, and after about 6 weeks I had gained/developed more fitness than I have in the last three years - and that coming off a COVID couch-potato spec body. The other day we did a climb in one go that I used to have to stop at least once or twice on, and while I still felt pretty gassed at the top - I recovered quickly and the rest of the ride wasn't ruined. Twice a week with lots of climbs is my plan where practicable going forward, because it makes the weekend (get a bit sketchy) ride a lot more enjoyable.
On a side note, I had a mate that spent a heap of time at the gym trying to gain some bike fitness. Total failure. He would monster the first climb, because he had the strength to mash up it, and then that was him done for the ride. Every climb after that was a mix of whining, whimpering or pushing. I've been there too (and will feel like I'm back there once I'm allowed back on the bike again), but IMO nothing but getting out there and grinding it out is going to do much for real-world riding fitness.