I know what you mean... I just went by what those phone apps always say, once I got them all setup (MapMyRide, MapMyRun, etc...)
57 yrs old, 6' 0", and 210 lbs (yeah I know, I'm a bit of a Clydesdale and fighting tooth and nail to lose those ten pounds!)

Over the last few years, I always do the annual "Monument Ave 10K" here in town (6.2 miles). The first time I did it, I about died and sat down in the grass at Monroe Park just past the finish line. It tears you up, bashes your knees and hips, and you are literally no good for the next day or two afterwards! Ouch...

But yet, I can ride 39 miles (my longest yet) in 2 hrs & 45 min (avg of 14 mph) and have a wonderful time! My knees and quads were a little tender afterwards, but certainly not like running a 10K!!!

Cycling is the best!!!