do you ride by time or miles?

[Fredwina]

If I understand correctly, if you go by time you will increase your miles in the same amount of time as your fitness improves?

Exactly!

IMakes me wonder if that is really true -- I guess you would have to do the same route on multiple days to determine that. Would you also monitor your HR to check fitness?

I monitor my HR when I remember to turn it on
To check the level I have several set courses. I then use my Bike journal account to comapare (did same route with high avg speed. lower avg hr, etc