If you have something like a garmin edge device, you can set an interval timer on it that will alert you every (say) 30 minutes cycling time which is what I have mine set to. Each time it goes off, I drink a few mouthfuls regardless of whether I feel I need it.
As for fluid/isotonic etc, go with what ever you find easiest to drink and if that is an isotonic drink, fine; if it is plain water fine. Don't worry about it too much, just drink before you are thirsty and make sure you carry enough liquid with you or can obtain more on route.
Dioralyte (or other ORS) can be useful to carry for emergencies (I have a medical condition called Addison's disease which means I can't balance salts properly, so dioralyte comes in handy from time to time, so have to be very careful).