Here’s a thought from a newbie himself (52) total uninformed, non medical professional perspective and if you have any concerns, you should absolutely see your doc.
But....
Maybe you’re just tired and need some sleep to recover. You do 30 miles after only riding for 8 weeks and you’re 67 years old. To me, it sounds like a huge workload for someone your age who has really just got started. Are you pushing yourself on these rides?
You ride in the morning, have a meal, and need a rest.
The fact that you are even riding such mileage without struggling during the ride would seem to indicate you are fine. And you are doing 20 miles with no issue after those rides.
If you took an average 67 year old (non cyclist)they couldn’t even do 5 miles and if they did more than 10, they would sleep for 3 days.
Ok, that’s my rant.
But seriously, if you have concerns, be safe and talk to your doc ; )