The highest scorers on the LSAT are math/physics majors, followed closely by philosophy majors. Presumably, these are also the highest achievers in law school. You have to study a lot for the LSAT, but you're in a good position to do very well. The LSAT is a very learnable exam, and the more you study, the higher your score will be.
When evaluating your chances for admission, keep in mind that your GPA is very impressive, especially for a math major. Admissions committees know that math is a much harder major than political science or English, so that will weigh in your favor. Another consideration is that, once you take the LSAT, the LSAC will send law schools a score report that includes the average GPA of all students in your undergrad. The lower that average, the stronger your 3.74 will appear.
As to performing well in law school, I wouldn't worry about your limited writing experience at all. On law school exams, you will have to resolve legal puzzles using terse logic. Exams are definitely not an exercise in eloquent writing, especially due to time constraints. Once you're a 2L and 3L, you might take some seminars that require more writing, but that will be your choice.
Math majors are in a very strong position to do well on the LSAT and in law school. I really believe it is one of the strongest majors you can apply with. The key, once you take the LSAT, is to learn everything you can about law school before it begins. It defintely makes sense to read as many books about law school as possible so you know what to expect once you start. You can also watch the Planet Law School DVD series -- the videos are very informative.