My method is to pick a distance, which is further than my last ride, or something I'd consider challenging. If the distance is 40 miles, I pick a destination approx. 20 miles away and cycle there. I then have to cycle 20 miles back, whether I like it or not
. I take a drink and a snack, stop when I think I need to and try and travel at a comfortable-but-not-killing-it pace on the way there to make sure I have enough energy to get back.
If I did a loop closer to home, I know I'd be tempted to give up and go home when I start to get tired or when my backside starts to hurt. I like to deprive myself of this option for my own good
. I've gone from being knackered after 10 miles to being knackered after 40-odd miles, so it seems to be working.