I'm gearing up to do several century rides this summer and hopefully put about 5,000 miles total on the bike (more if I achieve my car-free goal). I'm doing this on a shoe-string budget, so I won't be able to get any sort of power measuring device.

Currently my only metrics are current speed, max speed, and distance (as the average speed is way off because of city riding). I don't think I'll be able to get a HRM, possibly I can score a cadence meter though.

Most of what I have seen on training for improved performance has been on heart rate zones and intervals. I have been working on doing some HIITs, but not much more than powering on hard until I can't anymore than taking a very leisurely ride for a mile or so until I'm prepared to do it again.

Any tips on bettering performance without having a lot of data to work with?