I don't know tbh. When I first started cycling again a few years ago, I had a few loops of varying lengths which I did often and was forever trying to improve my time for them, getting depressed if I wasn't seeing an improvement. More recently, I don't care anymore, and poor road surfaces mean the days of really pushing it are over until (if!) they get fixed. I have come to realise that the time difference between a brisk but comfortable pace and absolutely flat out are very small and I am happy just to get the miles in and I have found my speed has improved without thinking about it. I did a ~40 mile ride on Sunday (longest in a few months), average of about 15.5mph on an old, heavy hi-ten Peugeot with steel wheels and cottered cranks. I realise many could go faster but I was pretty happy with that, mainly because I didn't feel I had really pushed myself so there must be more potential if I wanted.