I agree a computer on the bike is a must. A heart rate monitor can wait (unless oyur Doctor says you need one)
Once you have a computer you can map out a route and check your time, say a 20 mile loop. if your riding har and can cover that in say 1 hour and 15 minutes you can then try and cut the time to 1 hour 10 minutes and so one.
Riders trained for decades with nothing more than a wrist watch or stop watch.
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk