OK, did you route it like it was before you changed the belt. usually on the deck or up under the tractor there is diagram that shows the proper routing... If you are sure you routed it correctly then maybe the belt too big.

Usually if not paying attention routing the belt around the idler pulley (with the spring attached) get screwed up. when looking at the belt the v should be in the pulley grooves and the idler pulley usually FLAT and the back of the belt run around so the Flat part of the belt on the flat pulley.

The spring pulls the belt tight and when you push the brake peddle it loosen the belt thus stopping the motion of the tractor by detaching the engine from the rear transaxle..
If the tractor was moving before you worked on it I think if you study it you will find out the problem???