I let them lead at the start so I get an idea of their comfortable cruising pace. Once you've got this figured out, it's usually safe to take the lead for a while, but don't push the pace too hard and look back often to check that they're still on your wheel.

Use the ride as an exercise in cadence - shift down and spin as smooth as you can!

Sounds like in your case, the opportunities to ride with your dad are quite infrequent. If this is the case, make the ride about him not you. If it's 'too easy' for you, who cares?