When you race you will be utilizing what ever capacities you have built up to that day. In 5 weeks you are unlikely to move the aerobic threshold needle very much. The best plan will be to start a long aerobic base development period after the race when you have the time to concentrate on this. Run the race for sure and enjoy and learn from it. Don’t look at your HR and try to control it during the race. Obviously you need to pace yourself for an 8 hour race. But I don’t think you can or should worry about the AeT for this one.