I found a good technique for Highway Races if anyone's having trouble:
Equip your car with Aggro, find a racer, the easier the better, then drive ahead of them until you get an aggro slot, then wait a bit further ahead than them. Wait on the central reservation for them to come round, then when they pass, flash them, start the race. Then draft them. When you've got full draft boost, speed up behind them and hit the aggro just before you hit them. This will send them flying, and essentially put them out of the race. All you then need to do is dodge traffic - I find going on the edge is best, and use on/offramps to your advantage.