Take long trousers, hiking boots, waterproofs (both top and bottoms), hat, gloves, change of clothes back in the car.
You burn twice as many calories in the open air, so double up on your food. Take plenty of high energy food and slow burning carbohydrates. Take emergency food (chocolate bar, recommended for an energy boost).
Do you or someone you are with know where you are going? Ensure you have a compass & an ordinance survey map & you know how to use them. On a clear day hiking by sight is fine but the weather can change rapidly & you really don't want to get caught out. Respect the mountains. Are you walking alone? If so tell someone where you intend to hike, when and where you are starting out from, when you expect to finish and when you have finished let them know.