BetFair does not in fact provide any information about the race distance. We calculate this value using the clues in the market name, i.e. "6f" or "1m5f". There are no such clues in the name of the place market, so naturally the distance cannot be determined for that market.
What you can do is load win markets alongside with place markets and check the winplace_market_distance from inside the place market. Then if it does not match your criteria, you can delete this market, as well as the corresponding win market (there is a dedicated trigger action for deleting markets).