There are a few things to do to mitigate excess cable/mat:
1. The best advice is if you still can return the mat/cable to Amazon especially if you have a large amount of excess cable and get the next size system down.
2. Tighten spacing between cable spacing (keep at least 2 inches of space between heating cable) and DO NOT overlap ANY heating cables or heating cable over any other cable.
3. Remove cable from the mat and run it into areas where the mat does not fit. (Use a small amount of tape or glue to keep the heating cable in place.)
Reminder, DO NOT, UNDER ANY CIRCUMSTANCES, CUT (OR ATTEMPT TO SHORTEN) THE READ HEATING CABLE. The heating cable is specifically engineered to be the length it is.