If you’ve thought about running the Army Ten Miler this year, you have to make a decision fast.

There’s 35,000 spaces in the general registration that started last month, which sounds like a lot. But 31,484 spaces have already been taken as of June 1, according to the race’s website.

The race, which takes place this year on Oct. 7, is a beautiful 10 miles, starting at the Pentagon and taking runners through Arlington, Virginia, and Washington, D.C. It’s not an easy 10 miles to be sure, especially crossing back over to Arlington along a treeless, long bridge. But it’s worth the effort.

Military members and runners who’ve run the race at least seven times were given a special opportunity to register before the general registration started on May 23.