yup, pace and rhythm are likely to be your biggest problem. I find riding on multi use trails much more exhausting than finding a quieter road, as you have to concentrate much harder on the ped and bike traffic around you, and don't often get nice stretches where you can just cruise along at 20+ mph.
Repeating the same section over and over might get a little boring, too... but the advantages or rest areas and assistance nearby are certainly good to have.
Any organized century rides around that you can join?
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