[tomjakubowski]

A lot of people could get there with like six months of not that crazy training.

Cycling at 15mph on flat ground is pretty easy. If you can do that for an hour, and can progress at 10% increase in riding time week over week (pretty reasonable for someone who is still gaining fitness from "nothing"), you'll be doing four hour rides after just 16 weeks.

[missedthecue]

I would categorize 16 weeks of training as a lot of prep

[tomjakubowski]

I wouldn't - sixteen weeks is just about 1% of the 30 years you have between ages 20 and 50.

that said, ok, 15mph is not that easy -- maybe starting more like 10mph on a road bike for an hour would be more reasonable.

I still think most able bodied people could get to OP's fitness in much less than a year.

One data point - I was horribly out of shape in my mid 20s and got to riding 4 hour/15mph about four months after I bought my bike. But I was also unemployed for about a month of that!