Yes, you can input them manually. OpenFlights gives a a default number if you do nothing, but obviously actual distance flown differs. So I go to Flightaware to find that number, and enter it manually.
I'm now curious where OpenFlights gets that default number. It doesn't appear to align with gcmap.com figures.
Of course, gcmap is going to be lower than actual flight miles, because it's the shortest route, not actual.
Does Flightaware provide info for each flight, say, if it deviates from course it will be longer than usual?