It's normal to combine tenses like that. And, in fact, your sentence makes more sense than duo's. I've sent a comment to them explaining that 'learning' as it is used here requires a direct object, e.g. "....learned your your times tables...etc."
They're both correct and nearly synonymous, but ‘an den Strand gehen’ would tend to be used if that's your main destination, whereas ‘zum Strand gehen’ would tend to be used if it's just one stop in a larger itinerary.