The best way to leverage Marriott Points is to book their Hotel, Air, and Car packages where points are converted to miles at roughly 1:1. To directly convert points to miles, the best value is 125,000 points = up to 50,000 miles. To determine your best values per your needs based on the number of points you wish to use, navigate to: Home>Marriott Rewards >Use Marriott Rewards Points>Travel & Leisure or go directly to https://www.marriott.com/rewards/usepoints/morerew.mi.
If you must convert your points to air miles, be warned that you will lose more than two thirds of your points, it could be more depending on the airline. It gets better for 25,000 miles and higher but it is still not a 1:1 conversion. Go to Marriott rewards, then to travel rewards and order your conversion. For the air/ hotel travel reward, the required minimum points is now higher (starts at 200,000 points). Be warned that for the hotel part, if you only use partial stay (less than 7 days), you will lose the rest of it. It happened to me in Asia and to this day I still think it is unfair practice for Marriott.