I've now gotten to the point where I do at least one century per weekend, and have averaged 200 + miles for each of the last 3 weekends. I recently rode 168 miles in one day, and I have decided that's max for me because the thought of one more hour on top of that ride was not a happy prospect. The point is, though, that I got to the ability to do this by setting arbitrary goals for myself and not by comparing myself to other riders. I'm 57 years old--if I went on Strava and compared my times and elevations etc. to other riders, I'm sure I could find ones that would make me feel like my performance is inadequate. But I have to remember, if someone had told me 2 years ago I'd be riding these distances, I'd have thought they were nuts.

No one can tell you what is good for you--15 miles on a hilly, windy road is good if you're enjoying it and it makes you feel good. If you think you'll feel better doing another 5 miles, do it! If not, don't.

BTW, with the length of my rides, if I didn't vary them enough so that the longer ones didn't involve some exploring and/or a good meal, I'd go nuts out of boredom.

As to bikes--with distance, it isn't just how fast you can go. I had a '90s hybrid I could ride pretty fast, but after one of my arms swelled up from bad positioning combined with repetitive bumps after a 153 mile ride, I decided I needed either to ride less or get a better bike. I went with the FX 3 which is a fast, comfy flat bar. The day after I rode the 168 miles on it, I rode another 50 and felt fine. One thing I've learned the hard way is the value of proper fit of a bike grows exponentially with distance. Don't go too big or too small. I also think you can't buy a distance bike sight unseen and expect good results. You really need to try it out to see if the geometry lets you get into positions you can tolerate for hours.