Is riding on hilly road better than flats for long distance training? As you may know from my previous questions that I am training for a 150km ride. I want to know if riding on hilly road for 20km is better than riding on a flat road for 40km?
Better? Don’t know. Useful? Yes. 

One way you can use the hills to your advantage is to get your heart rate up. A good workout could be 4 minutes riding hard, then 3 minutes recovery, repeated 4-5 times. Riding hard means getting your heart rate up pretty high (171-180 for me) and this happens naturally and more consistently when you’re riding up hill. It’s also straightforward to do on the turbo.

https://www.ncbi.nlm.nih.gov/m/pubmed/17414804/

Maybe you can find a route with a few medium size hills that take a few minutes to climb, or ride up and down the same hill. 

If you warm up riding gently for 15-20 minutes before and warm down for 15 mins after, for example, then you’re likely to cover around 20km in around one hour. 

Of course it is still necessary to do a long ride every week for the training adaptations and to practise nutrition strategy, building up the distance weekly.