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!