I'm 67 and my cadence is generally between 75-85 which a comfortable range for me now. I do try to stay in the mid 80's but headwinds really slow me down these days, lol.

Getting a bike fit is a very good idea. I switched from a flat bar to a road bike because sitting straight up with the flat bar was really bothering my lower back. I got an "endurance" type road bike (Domane) and it really helped eliminate the sore back.

Also finally got a pair of cycling shorts which helps with the sore butt I used to get after 30 kilometers or so.

I think you just need to take the time to get back into your groove with cycling and accept that you may not be as strong or as fast as you once were. No biggie IMO, long as you're still riding, it's all good.