For the OP's (Wanderer) question?...increasing distance by 5 miles per ride when the first ride was 5 miles means you doubled and then tripled the distance in the first 3 rides...that alone is a daunting way to begin your season but at the same time?...

Great Job!...You sure are Fast-Tracking Things! LOL!

For some of the others here?...I can not express strongly enough just how important "SET-UP" is and also?..."PROPER BIKE SELECTION" (re: best suited for both you and the terrain)

While we're all roughly in the same age bracket?...we still have differing body shapes and sizes and?..."ABILITIES & DISABILITIES"

We need to embrace these facts of life and play to our remaining strengths...for instance?...while I'd love to romance the idea of me eventually logging century after century with me tucking into the drops on an aggressive road bike?...the reality for me is that attempting such would be self-defeating at best and once again put an abrupt end to my bicycling.

As such?...I've accepted that casual touring on a flat-bar hybrid is about as aggressive as I'm ever going to get and my rides will be gauged by quality & logging saddle time rather than the quantity of miles and how fast I peddled them.

There's a lot of great into, books and vids on "Proper Bike Set-up" but I've concluded that these are just good starting places and set-up should be a program of continual comfort improvement with great attention paid to saddle height & position with no hesitation given to making continual small incremental adjustments because it is the refining of those saddle adjustments that will both increase the comfort and quality of your ride and?...preserve those knee joints.

Hope I helped somebody and L8R, Bill.