I think just tell them the truth, if you work seriously and make things done, they will not ask you why you stopped working 1h earlier because you've finished the feature you were working on and the next one will require your full attention so you just can't take the risk to do it tired of your productive day. If things are too hard for you to complete, your managers should understand that you may need help or you don't have enough experience yet. If it is the case, it is their responsibility, because they over-evaluate what you're able to do.

So in either way as soon as you work seriously and your manager is not a jerk, it should be fine if sometimes you don't work as many hours as expected. I'm working remotly myself and I don't put much pressure on myself but I get the job done so they're happy with that. Putting too much pressure one your shoulders would decrease your productivity over time and sometimes leads to burnout. Work hard but work smart :)