Compute RTT in theoretical system
Why would it take longer than a few nanoseconds to get a message to the Moon and back? Because physics. Nothing can go faster than the speed of light.

Instead of packets on a network, think of suitcases on the baggage reclaim belt at an airport. The number of suitcases the airport workers put on the belt each second is the data rate; the length of time it takes a suitcase to go all the way around the belt is the round-trip time.

Suppose the airport employs more staff to throw luggage around. They can now put, say, twice as many suitcases onto the belt every second, but the belt doesn't move any faster so each individual bag still takes the same amount of time to go around.  They've increased the data rate but not the round-trip time. Because they're putting bags onto the belt more quickly, but the belt itself isn't moving any faster, there will be more bags in transit on the belt at any time.

Suppose that, instead of hiring more staff, they make the belt move twice as fast.  Now, you don't get any more bags per second but each bag only takes half the time to go around. They've decreased the round-trip time but not the data rate. Because bags now get delivered to waiting passengers faster, but they only appear at the original rate, there will be fewer bags in transit on the belt at any time.

So, you see that data rate and round trip time are essentially independent. The data rate is how many bits you can send each second, and the trip time is how long each individual bit takes to reach its destination.  You should now be able to figure out what the round trip time is, and also answer more complicated question like, "How long does it take to get a gigabyte of data to the moon on this link, and receive an acknowledgment that the final packet arrived?"